, (2^,  ~/2 


oip 

Illinois  state 
LIlBflMTORr  Of  ilORIll  RlSlOf 

URBANA,  ILLINOIS. 


TTMK 


Embryology  of  Limulus 


BY 

J.  S.  KINGSLEY. 


Reprinted  from  Journal  of  Morphology,  VoI.  VIL,  No.  i. 


BOSTON : 

& CONIPANY. 
1892. 


QINN 


THE  EMBRYOLOGY  OF  LIMULUS. 


J.  S.  KINGSLEY. 


CONTENTS. 


Historical 35 

Habits,  etc 36 

Methods 38 

Ovigenesis 40 

Early  Development 41 

Blastoderm  Cuticle 47 

External  Development 47 

Mesoderm 48 

Development  of  External  Form 49 

Comparisons 53 


Historical. 

The  embryology  of  Limulus  has  already  given  rise  to  con- 
siderable literature,  but  there  still  remain  many  problems  to  be 
solved.  The  first  to  describe  any  embryonic  stage  of  this  genus 
was  Henri  Milne-Edwards,  who  figured  and  briefly  described 
(’38,  ’39,  ’40)  a single  larval  stage.  The  next  was  Samuel  Lock- 
wood  (’70),  who  gave  an  account  of  the  oviposition,  and  the 
hatching  of  the  egg,  and  described  several  of  the  larval  stages. 
A.  S.  Packard  followed  with  a long  series  of  articles  (’70  ^ ’70 
’70 ’71,  ’72,  ’73,  ’75,  ’80,  ’85),  each  of  which  added  something  to 
our  knowledge.  Dohrn  (’7i),  who  studied  material  supplied 
by  Packard,  was  able  to  see  some  points  which  had  escaped  the 
latter.  Alexander  Agassiz  (’78)  described  the  habits  of  the 
young  after  the  beginning  of  a free  life,  and  contributed  a figure 
of  the  larva  to  Faxon’s  (’82)  compilation  of  drawings  illustrative 
of  the  embryonic  stages  of  Crustacea.  The  present  writer  made 
his  first  contribution  to  our  knowledge  of  the  development  of 
Limulus  in  1884,  following  it  in  October  of  the  next  year  with  a 
more  extended  paper  (’85).  In  the  same  month  H.  L.  Osborn 
(’85)  and  Brooks  and  Bruce  (’85)  published  preliminary  accounts 
of  their  investigations ; the  complete  papers  have  not  yet 

35 


36 


KINGSLEY. 


[VOL.  VII. 


appeared.  S.  Watase  has  made  the  structure  and  development 
of  the  visual  organs  the  subject  of  three  papers  (’89,  ’90®,  ’90 
William  Patten  (’89)  gave  a brief  note  on  the  origin  of  the  ner- 
vous system  and  sense  organs,  while  in  a later  paper  (’90),  in 
which  an  attempt  is  made  to  derive  the  Vertebrates  from  the 
Arachnids,  numerous  facts  relating  to  the  early  history  of  Limu- 
lus  are  given.  Lastly,  I have  presented  (’90)  a brief  abstract  of 
some  of  the  results  to  be  described  more  at  length  in  the  present 
series. 

Habits,  etc. 

The  American  horse-shoe  crab  (Limulus  polyphemu^  is  dis- 
tributed along  our  eastern  shores,  from  Maine  to  the  West  Indies 
and  the  Gulf  of  Mexico  (Vera  Cruz,  teste  J.  E.  Ives),  occurring 
at  certain  times  and  places  in  large  numbers.  Its  habits  have 
been  described  with  some  detail  by  Dr.  Lockwood  (’70).  Dur- 
ing most  of  the  year  it  frequents  deeper  water,  but  during  the 
breeding  season  — May  until  the  middle  of  July  — large  num- 
bers come  to  the  shore  for  the  purposes  of  oviposition.  I have 
never  been  able  to  notice  any  connection  between  the  hours 
when  they  frequent  the  shore  and  the  state  of  the  tide.  Sev- 
eral times  on  moonlight  evenings,  in  the  height  of  their  spawn- 
ing season,  I have  sailed  over  their  favorite  spawning  grounds, 
but  did  not  see  any  of  the  “crabs.” 

I do  not  know  where  the  couples  meet.  When  first  seen  they 
come  from  the  deeper  water,  the  male,  which  is  almost  always 
the  smaller,  grasping  the  hinder  half  of  the  carapax  of  the  female 
with  the  modified  pincer  of  the  second  pair  of  feet.  Thus  fas- 
tened together,  the  male  rides  to  shallow  water.  The  couples 
will  stop  at  intervals  and  then  move  on.  Usually  a nest  of  eggs 
can  be  found  at  each  of  these  stopping-places,  and  as  each  nest 
is  usually  buried  from  one  to  two  inches  beneath  the  surface  of 
the  sand,  it  appears  probable  that  the  female  thrusts  the  genital 
plate  into  the  sand,  while  at  the  same  time  the  male  discharges 
the  milt  into  the  water.  I have  not  been  able  to  witness  the 
process  more  closely  because  the  animals  lie  so  close  to  the  sand 
and  all  the  appendages  are  concealed  beneath  the  carapax.  If 
touched  during  oviposition,  they  cease  the  operation  and  wander 
to  another  spot  or  separate  and  return  to  deep  water.  I have 
never  seen  the  couples  come  entirely  out  of  the  water,  although 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


37 


they  frequently  come  so  close  to  the  shore  that  portions  of  the 
carapax  are  uncovered. 

I have  already  commented  upon  the  great  vitality  of  the 
eggs  and  the  young  (’85,  p.  522),  but  a few  words  more  may 
prove  of  interest.  When  studying  the  development  in  1884  the 
eggs  I studied  were  transported  200  miles  from  the  place  they 
were  laid.  They  were  six  days  5h  the  journey,  packed  in  moist 
sand,  but  without  any^ddition  of  salt  water.  On  August  i I 
left  the  shore,  taking  with  me  some  200  embryos  and  about  a 
pint  of  salt  water.  By  merely  supplying  the  loss  by  evapora- 
tion with  fresh  water  from  the  city  supply  I kept  some  of  these 
alive  until  November  20,  when  the  last  were  killed  to  supply 
material  for  study.  In  1890  I fertilized  some  eggs  on  June  22. 
Some  200  of  these  were  taken  in  half  a litre  of  water  to  Lin- 
coln, Nebraska,  1600  miles  from  the  shore,  where  they  lived 
from  September  7 to  the  week  of  November  14-20,  when  they 
were  killed  by  an  accidental  drying  up  of  the  water  during  a 
temporary  absence.  As  it  was,  they  lived  over  twenty  weeks  in 
confinement.  It  would  not  have  been  possible  to  keep  them 
much  longer,  as  the  stock  of  food  yolk  was  about  exhausted. 
Adult  specimens  have  been  shipped  alive  to  San  Francisco, 
and  now  one  meets  occasionally  with  notices  in  the  Pacific 
coast  papers  of  the  capture  of  horse-shoe  crabs,  probably  those 
planted  there  several  years  ago  by  the  U.  S.  Fish  Commission. 
They  have  also  been  shipped  alive  to  England  and  Germany. 
Professor  E.  Ray  Lankester  had  three  barrels  of  these  animals 
sent  him  in  London  from  Woods  Holl,  a large  proportion  of 
them  surviving  the  voyage.^ 

An  observation  made  by  Dr.  Lockwood  upon  the  retardation 
and  vitality  of  the  eggs  should  be  repeated.  He  says  (’70,  pp. 
271-272):  “At  the  close  of  the  warm  season  last  year  [1869] 
my  jars  must  have  contained  not  less  than  200  young  Limuli. 

. . . Hoping  to  continue  observations  upon  the  growth  of  my 
interesting  family,  the  jars  were  carefully  put  away.  Little 
regard,  however,  was  paid  to  temperature,  which,  on  several 
occasions,  went  down  to  the  freezing-point.  On  the  3d  of  May, 
1870,  I emptied  the  jars  to  see  how  my  charge  was  getting  on, 
when  lo  ! not  one  of  the  last  year’s  hatching  was  alive  ! but,  won- 

1 Mr.  Vinal  Edwards,  who  made  the  shipment,  informs  me  that  those  packed  with- 
out seaweed  or  other  moist  packing  survived  the  journey  the  best. 


38 


KINGSLEY. 


[VOL.  VII. 


derful  to  say,  at  least  a dozen  little  fellows,  all  hatched  this  spring, 
and  all  alive,  had  taken  their  place.  With  these  were  also  at 
least  thirty  eggs,  in  different,  but  all  in  advanced,  stages  of 
incubation.  In  some  of  them  the  young  could  be  plainly  seen 
revolving.”  Here  was  a retardation  of  development  for  almost 
a year ! 

Methods. 

The  observations  here  recorded  were  made  at  the  Marine 
Biological  Laboratory  at  Woods  Holl,  Massachusetts,  during 
the  months  of  June,  July,  and  August,  1889  and  1890,  and  in  the 
zoological  laboratory  of  the  University  of  Nebraska.  In  writing 
up  the  results  obtained  I have  been  hampered  not  a little  by 
my  distance  from  the  larger  libraries,  and  hence  the  compara- 
tive portion  of  the  paper  is  sadly  deficient  — a fact  which  no  one 
can  realize  more  than  myself. 

For  my  material  I have  relied  partly  upon  the  natural  nests 
and  partly  upon  artificial  impregnation.  With  the  former 
method  one  cannot  be  certain  of  the  age  of  his  material,  for  not 
infrequently  two  ovipositions  become  mixed.  I have  never  suc- 
ceeded in  getting  the  crabs  to  oviposit  naturally  in  confinement. 
In  artificial  impregnation  the  eggs  and  milt  were  sometimes 
obtained  by  squeezing  individuals  taken  in  copuloy  or  by  sucking 
these  products  from  the  genital  ducts  with  a pipette.  Very 
severe  squeezing  will  force  out  but  a small  number  of  eggs, — far 
fewer  than  are  naturally  laid  in  a nest, — while  any  attempt  to 
remove  them  from  the  body  by  cutting  covers  the  eggs  with  a 
layer  of  very  rapidly  coagulating  blood  {vide  Howell,  ’85),  which 
affords  an  excellent  nidus  for  bacterial  and  fungoid  growths. 

The  study  of  the  early  stages  has  proved  very  difficult  from 
the  fact  that  the  eggs  are  the  most  refractory  objects  I have 
ever  seen.  Until  the  outlining  of  the  germ  there  is  no  means 
of  orientation,  so  that  sections  must  be  taken  hap-hazard.  The 
greatest  care  must  be  taken  in  hardening  them  in  order  to  pre- 
vent the  yolk  becoming  too  hard  for  the  section  knife  ; and  after 
numberless  experiments  with  every  reagent  I could  think  of,  I 
came  to  rely  almost  entirely  upon  killing  the  eggs  by  heating 
them  in  sea-water  to  7o°~75°  C.  and  then  passing  them  through 
successive  grades  of  alcohol,  from  30  per  cent  to  70  per  cent,  in 
which  they  were  finally  kept.  Eggs  thus  treated  afforded  at 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


39 


the  moment  of  killing  excellent,  but  evanescent,  surface  views, 
as  a short  immersion  in  alcohol  renders  the  whole  surface 
one  uniform  color.  Hence,  in  order  to  orient  these  eggs  for 
subsequent  section,  I marked  each  one,  at  the  moment  of  killing, 
with  India  ink  — not  -affected  by  alcohol  — and  subsequently 
arranged  the  egg  with  reference  to  the  line  thus  afforded.  For 
staining  I used  chiefly  alum  cochineal  and  Grenacher’s  borax 
carmine,  while  a short  stay  in  osmic  acid  brought  out  certain 
details. 

I found  it  impossible  to  cut  the  early  eggs  in  paraffin. 
Absolute  alcohol  and  the  clearing  reagents  rendered  the  yolk 
extremely  hard  and  brittle,  while  the  paraffin  refused  to  pene- 
trate the  centre  of  the  egg.  So  for  the  early  stages  I had 
recourse  to  celloidin.  For  the  main  outlines  of  the  process 
employed  I am  indebted  to  the  suggestions  of  Dr.  H.  C.  Bum- 
pus.  The  celloidin  was  hardened  with  chloroform  and  cleared 
with  origanum  oil  or  with  a mixture  of  turpentine  and  carbolic 
acid  before  cutting.  The  sections  were  cut  with  the  knife 
flooded  with  the  clearing  fluid,  and  then  placed  in  order  on 
the  slide.  Being  already  cleared,  all  that  is  now  necessary  is 
to  apply  balsam  and  the  cover  glass.  In  many  respects  this 
process  is  identical  with  that  described  by  Eyclesheimer  (’90). 

To  study  the  stages  after  the  outlining  of  the  germ,  the 
chorion  was  removed  by  needles,^  and  then  by  careful  manipu- 
lation the  blastoderm  was  stripped  from  the  yolk,  stained,  and 
either  mounted  in  toto  for  surface  views  or  sectioned  as  usual  in 
paraffin.  In  the  later  stages  the  processes  of  development  so 
modify  the  yolk  that  the  whole  embryo  is  capable  of  being  sec- 
tioned in  the  usual  manner. 

As  a result  of  the  difficulties  of  manipulation  the  following 
account  of  the  early  stages  is  exceedingly  fragmentary,  yet  it  is 
hoped  that  the  little  here  detailed  will  prove  of  value,  especially 
as  almost  nothing  is  known  of  the  processes  involved  in  the 
formation  of  the  germ  layers.  (See  Postscript.) 

1 Owing  to  the  great  thickness  of  the  chorion  I found  it  difficult  to  control  the 
action  of  eau  de  Javelle  or  Labbaracque’s  solution.  Before  the  chorion  was  dissolved 
the  solution  would  frequently  affect  the  egg,  interfering  with  staining  and  making  it 
very  crumbly. 


40 


KINGSLEY. 


[VOL.  VII. 


Ovigenesis. 

I have  made  no  extended  observations  upon  the  origin  and 
development  of  either  eggs  or  spermatozoa.  The  gross  struc- 
ture of  the  ovaries  has  been  described  by  van  der  Hoeven  (’38), 
Gegenbaur  (’58),  and  Owen  (’72),  while  Gegenbaur  adds  a short 
account  of  the  origin  of  the  egg,  presented  in  abstract  by  Lud- 
wig (’75).  Owen  gives  a line  or  two  to  the  testis,  while  Benham 
(’83)  describes  it  more  in  detail.  Packard  (’72)  figured  the  sper- 
matozoa, Lankester  (’78)  noted  the  fact  that  they  are  motile,  and 
Packard  (’80)  refers  to  the  histology  of  the  testis  and  speaks 
briefly  of  the  development  of  the  ovary.  Aside  from  these  and 
one  or  two  older  papers,  at  present  inaccessible  to  me,  I 
know  of  no  published  results  upon  the  reproductive  organs  of 
Limulus. 

In  a female  Limulus  four  inches  long  (not  including  the  cau- 
dal spine)  I find  the  ovarian  caeca  lined  with  columnar  epithelium 
which  secretes  a delicate  cuticle,  and  outside  of  this  epithelium  a 
connective  tissue  tunica  propria.  As  in  other  higher  Metazoa, 
this  epithelium  is  the  ovogenetic  layer,  certain  of  its  cells  becom- 
ing modified  into  primordial  ova.  These  at  first  lie  within  and 
form  a part  of  the  parent  epithelium,  but  with  growth  the  eggs 
pass  to  the  outside  of  the  epithelium  and,  separating  the  tunica 
from  the  other  layer,  come  to  lie  between  the  two.  (Figs,  i 
and  2.) 

The  primordial  ova  are  distinguishable  not  only  by  their  size, 
but  by  their  more  deeply  staining  cytoplasm,  in  which  the  yolk 
spherules,  so  characteristic  of  the  mature  eggs,  are  lacking,  unless 
the  minute  granules  are  to  be  regarded  as  such.  Around  the 
cytoplasm  of  the  older  eggs,  after  leaving  the  epithelium,  there  is 
a delicate  membrane,  the  origin  of  which  I have  not  been  able  to 
decide,  but  I think  it  a true  vitelline  membrane.  The  nuclei  of  the 
ovarian  eggs  vary  considerably  with  age.  In  the  younger  ones 
they  are  strongly  staining  bodies  of  about  the  size  of  the  nucleoli 
of  older  eggs.  In  these  no  reticulum  is  visible.  A little  later  this 
nucleus  is  surrounded  by  a clear  space  which  separates  it  from 
the  darker  and  more  granular  cytoplasm.  This  clear  space  shows 
processes  radiating  into  the  surrounding  substance.  In  still 
older  eggs  a well-marked  nuclear  membrane  is  distinguishable, 
inside  of  which  is  a faintly  staining  chromatin  (i*)  reticulum 


No.  I.] 


THE  EMBRYOLOGY  OF  L/MULUS. 


41 


and  from  one  or  two  to  five  spherical  and  deeply  staining  nucle- 
oli. There  is  no  ‘ yolk  nucleus  ’ like  that  described  in  certain 
Arachnids. 

As  will  be  seen,  the  foregoing  description  differs  in  toto  from 
Packard’s  brief  account  and  figures  (’80,  p.  39,  PI.  IV,  Figs.  8, 
8^).  In  fact,  I cannot  determine  what  he  had  under  the  micro- 
scope. If  I understand  Gegenbaur  (’58)  aright,  the  eggs  in  his 
specimen^  project  into  the  lumen  of  the  ovarian  tube,  a differ- 
ence possibly  explicable  on  account  of  the  mature  condition  of  his 
material.  He  was  farther  unable  to  recognize  any  membrane 
around  the  egg  aside  from  the  epithelial  cuticle  of  the  ovarian 
tube.  In  other  respects  there  is  no  discrepancy  between  our 
accounts. 

Making  comparisons  now  with  the  Arachnida,  we  see  no  little 
similarity  in  the  structure  of  the  ovary  and  the  relations  of  the 
ova.  Metschnikoff  (’7i,  pp.  207-208,  PI.  XIV,  Figs,  i and  2) 
and  Laurie  (’90,  pp.  108-111,  PL  XIII)  describe  and  figure 
almost  the  same  condition  in  the  scorpion.  The  ovary  consists 
of  the  same  epithelium  and  tunica,  and  the  eggs,  as  they  increase 
in  size,  come  to  lie  between  these  two  layers.  The  differences  are 
that  in  Limulus  each  egg  is  not  enveloped  in  a separate  follicle  ; 
but  in  the  scorpion,  where  the  eggs  are  few,  such  is  the  case. 
In  Limulus  the  epithelium  does  not  form  such  a well-marked 
“stalk”  connected  with  the  egg  as  in  the  scorpion;  and  the 
cells  of  this  stalk  are  columnar,  not  stratified.  Closely  similar 
resemblances  can  be  traced  with  the  Araneida,  as  epitomized  by 
Ludwig  (’75)  and  the  Acarina  (Pagensticher,  ’60-’6l).  In  the 
Crustacea,  on  the  other  hand,  a similar  condition  is  not  found, 
there  being  nowhere  an  ovary  with  a similar  constitution.  In 
short,  so  far  as  my  observations  on  ovigenesis  go,  Limulus 
agrees  well  with  the  Arachnida  and  contrasts  strongly  with  the 
Crustacea. 

Early  Development. 

The  eggs  of  Limulus,  as  they  come  from  the  oviduct,  vary  con- 
siderably in  size  and  shape.  They  are  usually  more  or  less  oval, 

1 Twenty-five  German  inches  long.  Gegenbaur  is  in  doubt  about  his  specimen. 
In  appearance  it  was  clearly  L.  molluccanus,  but  so  far  as  he  was  able  to  find  out  it 
came  from  the  West  Indies.  As  Z.  polyphemus  and  Z.  molluccanus  are  easily  distin- 
guishable, it  is  possible  that  a mistake  was  made  in  locality. 


42 


KINGSLEY. 


[VOL.  VII. 


being  somewhat  flattened  at  first  by  mutual  pressure  in  the  ovi- 
duct. The  average  diameter  is  perhaps  two  millimetres.  Each 
egg  is  enveloped  in  a tough  chorion  in  which  a laminated  struc- 
ture is  readily  recognizable.  I have  never  been  able  to  discover 
any  opening  or  pores  in  this  chorion  through  which  impregna- 
tion can  be  effected,  although  it  is  certain  that  fertilization  must 
take  place  outside  the  body  of  the  female,  and  hence  after  the 
chorion  is  formed.  The  egg  proper  consists  of  a large  mass  of 
strongly  refractive  yolk  globules  of  various  sizes,  and  in  the  egg 
as  it  comes  from  the  oviduct  I have  been  unable  to  find  a trace 
of  a nucleus,  nor  of  nuclear  material.  No  matter  what  stain 
was  employed,  I could  not  recognize  any  chromatin  granules 
scattered  through  or  upon  the  yolk,  while  anything  that  might 
be  considered  as  protoplasm  was  very  scanty. 

In  this  my  experience  is  paralleled  by  that  of  certain  other 
students  of  Arthropod  eggs.  The  nucleus  can  be  traced  to  a 
certain  stage  of  ovarian  development  where,  as  Stuhlmann 
says  (’86),  “ Spater  verschwindet  das  Keimblaschen  vor  unseren 
Blicken,  bis  wir  endlich  am  oberen  Eipol  der  Furchungskern 
wieder  finden.”  Of  course  this  absence  is  apparent  rather  than 
real,  as  has  been  shown  by  numerous  other  observations. 

I have  been  equally  unsuccessful  in  my  attempts  to  witness 
the  phenomena  of  fertilization,  nor  have  I seen  any  features 
undoubtedly  characteristic  of  maturation,  although  I have  sec- 
tioned many  eggs.  In  one  egg,  an  hour  after  fertilization,  I 
found  on  one  side  a faintly  staining  structure  which  I have 
possibly  thought  may  have  been  a polar  globule  (Fig.  3),  but  the 
fact  that  a nuclear  stain  brought  out  no  chromatin  inside  the 
yolk  renders  this  doubtful. 

The  various  steps  of  development  vary  in  time,  not  only  with 
the  temperature,  but  with  eggs  of  the  same  lot  exposed  to 
exactly  the  same  conditions.  Hence  the  ages  quoted  in  the 
following  pages  must  be  understood  as  averages.  Thus,  in  one 
lot  of  eggs  I have  found  phenomena  occurring  at  four  hours, 
which  in  others  occurred  at  twenty-four  hours,  while  in  later 
stages  there  may  be  variations  of  a month  or  more. 

At  the  time  of  impregnation,  the  surface  of  the  egg  is  covered 
with  dark  yolk  granules,  each  granule  having  a lighter  boun- 
dary. The  granules  vary  in  size,  and  the  egg  completely  fills 
the  chorion.  In  fifteen  minutes  the  chorion  distends  so  as  to 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


43 


leave  a space  between  it  and  the  egg,  and  at  the  same  time  its 
outline  becomes  regular  and  ellipsoidal.  In  half  an  hour  the 
granules  begin  to  break  up  and  become  smaller,  while  the  yolk 
begins  to  swell,  and  at  the  end  of  an  hour  completely  fills  the 
chorion. 

In  four  hours  begin  those  strange  modifications  of  the  sur- 
face already  noticed  by  H.  L.  Osborn  (’85)  and  by  Brooks  and 
Bruce  (’85).  Viewed  from  the  surface  the  eggs  exhibit  a num- 
ber of  fissures,  usually  at  one  pole  of  the  egg,  which  strongly 
simulate  cleavage  furrows  (Figs.  4,  5,  6).  I have  not  be  enable 
to  kill  such  eggs  quickly  enough  to  preserve  these  furrows  for 
section.  Even  when  dropped  into  hot  water  the  surface  would 
become  smooth  before  death  ensued. 

Sections  of  such  eggs  present  some  features  difficult  of  inter- 
pretation. In  the  earlier  phases  near  one  pole  there  appears 
a clear  line  inside  the  yolk,  concentric  with  the  surface,  which 
marks  off  a central  from  a superficial  portion ; while  in  older 
eggs  (Fig.  7)  the  line  has  extended  nearly  around  the  egg. 
Inside  of  this  line  were  no  features  worthy  of  mention,  and  in 
the  several  eggs  sectioned  no  nucleus  was  to  be  found.  Out- 
side of  the  line  the  yolk  becomes  broken  up  into  numbers 
of  columnar  bodies  — like  the  cells  of  columnar  epithelium  — 
with  rounded  external  ends.  These  yolk  columns  are  separated 
from  each  other  by  a slightly  staining  protoplasm  (Fig.  8), 
and  the  outer  ends  of  these  columns  are  more  free  from 
yolk  spherules  than  are  the  deeper  portions.  I think,  notwith- 
standing the  apparent  disparity  of  dates,  that  it  was  an 
early  stage  of  this  process  which  Brooks  and  Bruce  describe 
when  they  say  of  an  egg  of  twenty-four  hours  “ protoplasmic 
processes  or  pseudopodia  extend  from  the  [protoplasmic]  cap 
into  the  yolk,  and  surrounding  and  including  the  substance  of 
the  yolk  divide  this  up  into  a number  of  yolk  balls.”  After 
a short  time  these  motions  of  the  external  surface  cease,  and 
the  egg  becomes  as  smooth  as  before,  while  in  section  no 
change  is  recognizable  except  that  there  is  a thin  layer  of 
protoplasm  — a true  blastema  ^ — over  the  whole  yolk. 

1 As  I have  already  indicated  (’86,  p.  116,  foot-note),  I use  the  term  blastema 
in  the  original  sense.  Patten . defines  (’84,  p.  564)  the  blastema  as  “a  thin  nucle- 
ated layer  of  protoplasm  covering  the  whole  outer  surface  of  the  yolk,  and  not 
divided  into  distinct  cells.”  He,  however,  suggests  that  it  is  not  impossible  that  a 


44 


KINGSLEY. 


[VOL.  VII. 


I am  in  doubt  as  to  the  interpretation  of  these  phenomena. 
They  are  not  connected  with  segmentation.  Two  possibilities 
have  suggested  themselves.  One  is  that  they  may  possibly 
be  compared  with  those  still  unexplained  polar  rings  described 
by  Whitman  (’78,  p.  234),  on  both  poles  of  the  maturing  egg 
of  Clepsine  (a  suggestion  of  doubtful  value).  The  other  would 
view  them  as  connected  with  the  formation  of  the  blastema. 
It  is  certain  that  a blastema  surrounds  the  egg  of  Limulus  after 
this  process  while  none  was  visible  before. 

In  one  egg  of  about  twelve  hours  I found  what  I regarded  in 
my  preliminary  paper  (’90)  as  the  segmentation  nucleus,  occupy- 
ing a subcentral  position  in  the  yolk,  but  I have  not  succeeded 
in  connecting  it  with  the  later  stages.  In  other  eggs  of  the 
same  age  I find  a thickening  of  the  blastema  on  one  side  of 
the  egg,  but  no  stain  serves  to  distinguish  a nucleus  in  it,  but 
still  it  may  be  present.  The  position  of  the  segmentation 
nucleus  has  no  great  taxonomic  importance,  as  in  both  Crustacea 
and  Arachnida  it  may  be  either  subcentral  or  superficial.^ 

Stage  A.  — Between  twelve  and  twenty  hours  I have  not 
been  able  to  get  any  sections  showing  anything.  At  twenty 
hours  I found  an  egg  containing  eight  nuclei.  By  drawing 
these  in  their  relative  positions  and  projecting  them  on  a plane 
(Fig.  9),  a marked  polarity  in  their  distribution  is  apparent. 
As  will  be  seen,  the  nuclei  are  much  nearer  to  one  pole  of 
the  egg  than  to  the  other,  and  had  the  plane  of  projection 
been  slightly  different  this  polarity  would  have  been  more 
marked.  This  condition  is  intelligble  on  the  view  that  the 
segmentation  nucleus  is  subcentral  as  well  as  if  it  be  regarded 
as  superficial. 

In  the  next  twenty  hours  there  are  no  phenomena  to  detail  at 
length.  From  the  surface  no  changes  are  visible,  while  sections 
reveal  a gradual  increase  in  the  number  of  nuclei,  the  polarity 
just  mentioned  persisting  in  their  distribution. 

blastema  may  exist  without  nuclei.  The  term  blastema  was  first  used  by  Weismann 
(’63)  for  a non-nucleated  layer  in  Musca  and  Chironomus,  and  such  a layer  has 
been  shown  to  exist  in  many  eggs  by  various  authors,  among  them  Metschnikoff 
(’66)  in  Aphis,  Aspidotus,  Csecidomyia;  Witlaczil  (’84)  in  Aphis;  Locy  (’86)  in 
Agalena;  Heider  (’89)  in  Hydrophilus;  Voeltzkow  (’89)  in  Musca,  etc.  A 
blastema,  then,  is  a layer  of  anucleate  protoplasm  around  the  yolk. 

1 E.g.  subcentral  in  Cetochilus  (Grobben,  ’81),  Crangon  (Kingsley),  Eupagurus 
(Mayer,  ’77),  Porcellio  (Reinhard,  ’87),  Araneina;  peripheral  in  Nebalia  (Metschni- 
koff),  Mysis  (Van  Beneden,  ’69),  Scorpio  (Laurie),  Acarina  (Claparede,  ’68). 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


45 


In  from  forty-two  to  forty-eight  hours  the  eggs,  as  they  lie  in 
the  dish,  show  on  their  upper  surfaces  the  first  traces  of  segmen- 
tation of  the  yolk.  In  this  there  is  no  regularity  as  to  the  direc- 
tion of  the  furrows  nor  uniformity  in  their  extent.  At  first 
the  furrows  are  clean  cut,  with  well-defined  margins  and  some 
depth,  but  soon  they  become  shallower,  and  the  margins  and  bot- 
toms become  irregular  by  the  formation  of  numerous  yolk  spheres 
of  varying  size  (Figs.  lo,  ii).  Gradually  the  furrows  flatten  out, 
and  the  yolk  spheres  become  merged  in  the  general  yolk  of  the 
surface,  and  the  egg  is  as  smooth  as  before.  In  from  four  to  six 
hours  this  process  is  repeated,  the  spaces  between  the  furrows  be- 
coming smaller  and  the  furrows  embracing  more  of  the  egg  than 
before  (Fig.  12).  This  is  repeated  several  times,  until  at  last 
the  whole  surface  is  included  in  the  segmentation  (Figs.  13,  14). 
At  each  of  these  divisions  there  are  at  first  the  same  clean-cut 
furrows  followed  by  the  same  irregularity,  and  eventually  by  the 
apparent  obliteration  of  the  planes  of  segmentation. 

Sections  plainly  show  (Fig.  15)  that  this  is  a true  segmenta- 
tion of  the  yolk,  the  result  being  to  divide  the  whole  egg  into  a 
series  of  cells,  each  consisting  of  a mass  of  yolk  (Fig.  18)  with 
a central  nucleus.  It  is  also  apparent  that  therewith  is  con- 
nected the  appearance  of  the  nuclei  at  the  surface  of  the  egg 
and  the  formation  of  a blastoderm  (Fig.  15).  In  the  projection 
of  an  egg  of  forty-eight  hours  (Fig.  16)  twenty-six  nuclei  were 
seen.  A little  later  {2\  days)  a higher  power  shows  some  inter- 
esting phenomena.  The  nucleus  is  surrounded  by  an  amoeboid 
mass  of  protoplasm,  sending  processes  into  the  surrounding  yolk, 
while  the  planes  of  segmentation,  as  well  as  the  external  surface 
of  the  egg,  are  covered  with  a thin  layer  of  faintly  staining  pro- 
toplasm (Fig.  17),  apparently  the  blastema  of  the  earlier  stages. 
At  the  time  when  these  furrows  seem  to  disappear  {suprd)^  this 
protoplasm  regains  the  surface,  but  the  furrows  themselves 
remain,  and  eventually  the  whole  egg  is  divided  into  nucleated 
yolk  cells  (Fig.  18). 

At  first  the  central  portion  divides  as  rapidly  as  the  peripheral, 
and  in  each  portion  of  the  egg  the  cells  are  about  equal  in  size ; 
at  last,  however,  the  central  cells  enter  upon  what  may  be  called 
a resting  stage,  which  condition  persists  until  after  the  beginning 
of  a free-swimming  life.  Their  divisions  occur  at  infrequent 
intervals,  and  the  differences  in  size,  from  the  appearance  of 


46 


KmGSLEY. 


[VoL.  VII. 


the  germ  until  the  caudal  spine  appears,  are  scarcely  notice- 
able. 

Stage  B.  — The  peripheral  cells,  on  the  other  hand,  divide 
more  rapidly,  so  that  in  five  days  from  impregnation  (Fig.  19) 
there  is  a marked  difference  between  the  cells  on  the  surface  and 
those  deeper  in  the  egg.  A more  careful  study  shows  that  this 
division  of  the  surface  cells  has  a peculiar  character.  In  each 
instance  (see  Fig.  20,  which  represents  a portion  of  an  egg  of  5J 
days)  the  first  division  of  the  peripheral  cells  occurs  in  a plane 
parallel  to  the  surface  of  the  egg.  This  is  plainly  shown  in  the 
cases  of  the  cells  marked  where  the  direction  of  the  mitotic 
spindle  shows  the  direction  of  the  future  division  — a view  which 
is  confirmed  by  a study  of  the  other  cells.  Another  feature  is 
noticeable.  The  products  of  this  division  are  unequal.  There 
is  a deeper  and  larger  cell  containing  a large  amount  of  food 
yolk  and  closely  resembling  the  neighboring  yolk  cells ; and  a 
superficial  smaller  and  flattened  cell,  richer  in  protoplasm  and 
containing  far  less  yolk.  In  this  way  a blastoderm  is  differen- 
tiated, but  the  process  has  in  my  opinion  a deeper  significance, 
for  by  it  the  entoderm  is  separated  from  the  rest  of  the  egg. 
In  other  words,  in  Limulus  the  two  primary  germ  layers  are 
differentiated  by  multipolar  delamination. 

This  process  is  clearly  allied  to  that  multipolar  delamination 
which  Morgan  (’90,  ’91)  has  described  as  occurring  in  the  eggs  of 
certain  pycnogonids  and  Faussek  (’9l)  in  phalangids.  While  I 
shall  discuss  it  later,  I may  say  here,  that  it  probably  has  at 
most  a very  distant  relationship  to  the  Coelenterate  delamina- 
tion, but  has  arisen  within  the  Arthropod  phylum. 

After  the  formation  of  the  blastoderm,  i.e.  the  separation  of 
ecto-mesoderm  from  entoderm,  I have  not  been  able  to  add 
much  to  our  knowledge  until  about  eight  days  after  impregna- 
tion. The  absence  of  all  features  which  would  aid  in  the  orien- 
tation of  the  egg  makes  it  necessary  to  cut  all  sections  at 
random,  while  the  opacity  renders  surface  views  impossible. 
In  general  this  time  is  occupied  by  a multiplication  of  the 
blastoderm  cells  and  a consequent  diminution  in  their  size. 
This  multiplication  by  division  proceeds  at  a more  rapid  rate  at 
one  pole  of  the  egg  than  at  the  opposite,  the  result  being  that 
soon  a germinal  pole  may  be  recognized  by  the  smaller  and 
more  columnar  cells,  those  at  other  portions  retaining,  until 
later  stages,  more  the  appearance  of  pavement  epithelium. 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


47 


Blastoderm  Cuticle. 

With  the  formation  of  the  blastoderm,  the  blastodermic  cuticle 
is  first  laid  dow-^.  Its  history  need  not  be  given  here,  as  I have 
already  (’85,  p.  524)  detailed  it,  and  have  suggested  for  it  and  sim- 
ilar envelopes,  ClaparMe’s  term  ‘‘deutovum.”  The  occurrence 
of  these  Blastodermhauten  is  frequent  in  the  Arthropod  phylum. 
In  Limulus  the  envelope  persists  as  a protective  structure  until 
a late  stage  in  development,  but  it  is  omitted  from  my  figures. 

Early  External  Development. 

Stage  C.  — At  from  six  to  eight  days  after  impregnation  a 
lighter  patch  is  visible  on  one  side  of  the  egg.  Its  outline  is 
not  distinct,  but  in  general  it  may  be  said  to  be  circular.  The 
change  which  this  undergoes  in  two  or  three  days  (eight  to 
eleven  days  from  impregnation)  is  slight ; at  the  latter  date  a 
pit  is  apparently  seen  in  the  centre  of  the  white  spot  (Fig.  21). 

For  this  lighter  patch  I have  taken  the  same  name  which 
was  given  by  Claparede  (’62)  to  a similar  structure  in  the  develop- 
ing Arachnid  egg.^  The  spot  is  the  first  appearance  of  what  is 
to  form  the  primitive  streak.  At  first  this  spot  is  circular, 
but  it  soon  becomes  elongate.  The  next  day  a second  cloud 
appears  immediately  adjoining  the  first  and  connected  with  it 
(Fig.  22).  I am  not  positive  in  my  identification,  but  believe 
that  the  primitive  cumulus  marks  the  anterior  end  of  the 
embryo.  At  first  the  posterior,  or  secondary,  cloud  is  smaller 
than  the  primitive  cumulus,  but  it  rapidly  increases  in  size, 
while  its  outlines  become  more  indistinct  than  shown  in  Fig.  23. 
At  the  same  time  the  primitive  streak  extends  backward  from 
the  spot  mentioned  above,  into  the  posterior  cloud  ; the  anterior 
spot  remaining  the  widest  of  the  whole.  For  reasons  which 
will  appear  farther  on  I regard  the  widest  end  of  the  primitive 
streak  as  marking  the  position  of  the  future  mouth.  The 
posterior  cloud  continues  to  grow  until  the  result  is  as  shown 
in  Fig.  24. 

Next  there  appears  a transverse  line  behind  the  primitive 
cumulus,  cutting  the  embryo  into  an  anterior,  or  cephalic 

1 I retain  this  term,  ‘ primitive  cumulus,’  notwithstanding  Kishinouye  (’90)  has 
shown  that  it  is  possible  that  Claparede  has  mistaken  the  order  of  appearance  of 
his  “ cumulus  primitif  ” and  the  ‘ calotte.’ 


48 


KINGSLEY. 


[VOL.  VII. 


region,  and  a posterior  or  thoracico-abdominal  portion,  the  ce- 
phalic being  the  smaller  and  more  sharply  differentiated  from 
the  rest  of  the  blastoderm.  This  occurs  on  the  average  about 
fifteen  days  after  impregnation.  Twelve  hours  later  a second 
line  occurs  behind  the  first,  cutting  off  from  the  thoracico- 
abdominal  region  the  first  somite  of  the  body ; and  about  twelve 
hours  later  a third  transverse  line  appears  (Fig.  25),  and  now 
there  is  a head  region,  two  body  somites,  and  an  undifferentiated 
caudal  region. 

This  figure  (Fig.  25),  taken  from  a blastoderm  peeled  from  the 
egg  and  mounted  in  balsam,  shows  clearly  that  this  appearance 
of  somites  and  lines  of  separation  results  from  the  fact  that  the 
cells  are  abundant  in  certain  regions  and  more  scattered  in 
others ; in  other  words,  from  the  outlining  of  the  mesodermic 
somites.  This  process  continues  until  six  segments  behind 
the  head  are  formed,  the  sixth  consisting  of  the  united  sixth 
‘ thoracic  ’ segment  and  the  caudal  plate.^  At  first  these  seg- 
ments are  quite  short  and  correspondingly  broad  (Fig.  26),  but 
later  they  increase  rapidly  in  length.  I may  say  in  passing  that 
owing  to  the  difficulties  of  observation  it  was  not  possible  to  be 
certain  of  the  limits  and  proportions  in  certain  figures.  Each 
egg  had  to  be  studied  in  strong  sunlight,  and  the  use  of  a 
camera  was  impossible.  Such  was  the  case  with  Figs.  22, 
23,  26. 

Mesoderm. 

The  primitive  cumulus  is  shown  in  section  in  Fig.  36.  As 
will  be  seen,  the  surface  of  the  egg  is  covered  with  a layer  of 
thin,  flattened  cells,  while  beneath  are  the  entoderm  cells.  The 
cumulus  itself  is  thicker,  partly  owing  to  the  fact  that  its  com- 
ponent cells  are  more  columnar,  and  also  to  the  fact  that  lower 
layer  cells  have  been  formed.  The  spot  in  the  cumulus,  which 
in  surface  views  (Fig.  21)  looks  like  a pit,  is  seen  in  sections  to 
be  produced  by  the  greatly  thickened  centre  of  the  cumulus. 

1 In  a few  instances  I have  seen  reason  to  doubt  this.  In  almost  every  instance 
I have  seen  all  six  appendages  arise  at  the  same  time,  but  in  two  or  three  cases, 
{e.g.  Fig.  28)  but  five  appendages  appear  at  first,  the  appendage  i being  noticeable 
at  a later  date.  This  may  indicate  that  the  corresponding  segment  may  be  corre- 
spondingly delayed;  and  that  the  above  interpretation  is  not  correct.  On  the 
other  hand,  these  instances  may  belong  to  some  of  the  many  anomalies,  which 
are  found  in  examining  a large  series  of  Limulus  embryos. 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


49 


Sections  of  the  embryos  shown  in  Figs.  21-23  show  but 
slight  differences  from  those  of  Fig.  24,  and  hence  a descrip- 
tion of  that  will  suffice.  Fig.  37  represents  a section  through 
the  anterior  end  of  the  streak  at  stage  C.  In  the  median  line 
is  the  streak  itself,  which  shows  a median  proliferation  of  cells 
extending  some  distance  into  the  yolk,  while  on  either  side  is  a 
less  conspicuous  thickening  of  the  blastoderm.  . All  three  of 
these  elements  enter  into  the  formation  of  the  appearance  of  a 
primitive  streak  as  viewed  from  the  surface,  and  all  contribute  to 
the  formation  of  the  middle  germ  layer.  At  the  point  of  the 
section  the  median  ridge  extends  below  the  others,  and  the 
nuclei  at  its  inner  extremity  show  a tendency  to  spread  towards 
right  and  left.  Farther  back  (Fig.  38)  these  same  centres  of 
proliferation  may  be  traced,  and  here  the  lateral  as  well  as  the 
median  band  contributes  to  the  mesoderm.  From  the  primitive 
streak  the  mesoderm  here  extends  right  and  left  to  the  margin 
of  the  germinal  area,  where,  apparently,  it  again  connects  with 
the  ectoderm.  In  some  sections,  especially  in  later  stages,  other 
points  of  connection  occur  between  ecto-  and  meso-derm,  but  I 
have  not  been  able  to  trace  any  regularity  in  these. 

This  account  accords  well  with  that  of  Patten  (’90),  except 
that  I have  failed  to  trace,  in  surface  view,  the  ring  of  mesoderm 
extending  completely  around  the  embryo  to  which  he  refers 
(P-  375)'  Probably  this  is  represented  by  the  marginal  con- 
nection between  ectoderm  and  mesoderm  in  my  figure. 

In  this  method  of  mesoderm  formation  a portion  of  the  periph- 
eral part  of  the  yolk  is  cut  off  by  the  outgrowing  middle  layer, 
and  comes  to  lie  between  it  and  the  ectoderm  (Fig.  39),  This 
yolk  is  in  such  position  that  it  can  readily  serve  as  food  for  the 
growing  ectoderm,  and  although  I have  no  evidence  on  this  point, 
I believe  that  such  is  its  fate. 

The  subsequent  history  of  the  mesoderm  and  its  derivatives 
will  be  followed  in  detail  in  the  next  portion  of  these  studies. 

Development  of  External  Form. 

Stage  D.  — The  next  step  is  the  formation  of  the  appendages. 
So  far  as  my  observations  go  this  process  would  seem  to  take 
place  nearly  simultaneously  on  all  of  the  cephalothoracic  post- 
oral  segments  in  the  majority  of  eggs.  Yet  this  is  not  the  case 


KINGSLEY. 


[VOL.  VII. 


SO 

in  all.  Figs.  27  and  28  show  two  modifications  which  I have 
witnessed,  the  latter  in  two  instances.  In  the  first  and  more 
normal  of  these  figures  the  cephalic  region  is  small,  and  behind 
it  come  six  somites,  each  with  the  outline  of  a pair  of  appen- 
dages. The  sixth  appendage  is  faint,  and  the  segment  which 
bears  it  is  not  yet  differentiated  from  the  abdominal  region. 
The  same  state  of  affairs  is  shown  in  the  slightly  later  stage 
represented  in  Fig.  29,  made  from  an  embryo  peeled  from  the 
egg,  and  which  also  shows  several  other  points  to  be  described 
later.  In  this  the  mesodermic  somites  are  obscured,  while  the 
abdominal  region  is  more  elongate.  On  the  other  hand.  Fig.  28, 
also  made  from  a transparent  specimen,  shows  but  jive  pairs 
of  thoracic  feet,  while  in  other  respects  the  embryo  is  much 
further  advanced,  as  is  shown  by  the  existence  of  appendages 
VII  and  VIII  (operculum  and  first  gill-bearing  appendage)  in 
the  abdominal  region. 

This  variation  in  the  time  and  order  of  the  appearance  of  the 
appendages  probably  explains  the  difference  between  Dohrn 
(’71)  and  Packard,  the  former  stating  that  appendage  I appears 
later  than  the  others.  This  is  certainly  true  in  some  cases,  but 
out  of  several  hundred  eggs  examined  at  about  the  time  of  the 
appearance  of  the  feet,  I have  seen  but  two  instances,  and  in 
my  former  papers  (’85  and  ’90)  I took  Packard’s  position,  as  at 
the  times  when  those  papers  were  written  I had  not  seen  a 
specimen  without  appendage  I. 

Professor  H.  L.  Osborn  (’85,  p.  2)  gives  the  following  account 
of  the  appearance  of  the  limbs  in  Limulus : “ On  July  28th,  1 1.3Q 
A.M.  [the  eggs  were  fertilized  July  23],  a deep  semicircular  de- 
pression showed  itself.  On  the  29th,  in  the  space  between  the 
two  lips  of  the  depression  of  the  day  before,  a pair  of  buds  ap- 
peared — the  beginnings  of  the  anterior  pairs  of  limbs.  On  the 
following  day  two  more  pairs  are  added,  and  in  front  of  the  first 
pair  and  behind  the  front  lip  of  the  fold  a most  important  struc- 
ture is  for  the  first  time  seen : it  is  a slit  elongated  antero- 
posteriorly, — the  definitive  mouth  opening.  It  is  distinctly  in 
front  of  the  first  pair  of  limbs.  It  is  to  be  noted  that  the  anal 
opening  has  not  yet  shown  itself,  according  to  my  observations. 
The  stomodaeum  and  the  three  somites  are  now  included  in 
an  area  which  is  plainly  marked  off  from  the  rest  of  the  egg 
and  surrounded  by  an  oval  elevation.  On  the  following  day, 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


51 


July  31,  there  had  appeared  inside  this  rim  the  remaining  pairs 
of  cephalothoracic  appendages.” 

Although  I have  looked  carefully  for  the  appearances  thus 
described,  I cannot  confirm  the  description.  Still,  there  are  so 
many  anomalies  in  the  history  of  many  eggs  that  it  is  possible 
that  the  conditions  ■v\ritnessed  by  Professor  Osborn  may  some- 
times occur.  For  instance,  in  some  eggs,  after  the  somites  are 
partially  outlined,  a deep  longitudinal  groove  appears,  transverse 
to  the  somites  and  extending  the  whole  length  of  the  embryonic 
area.  The  lips  of  this  groove  sometimes  even  touch  each  other, 
and  in  the  tube  thus  formed  the  limbs  bud  out.  Again,  in  other 
eggs  a deep  invagination  may  take  place  in  the  abdominal  region, 
carrying  in  with  it  the  abdominal  feet.  Such  eggs  appear  later 
to  regain  the  normal  appearance  and  to  develop  in  the  regular 
manner. 

Concerning  the  later  features  of  external  development  but 
little  needs  to  be  said.  The  figures  given  by  Packard  Dohrn, 
and  myself  are  sufficient  to  indicate  most  of  the  features  of  the 
growth  of  body  shape  and  the  positions  and  changes  of  forms  of 
the  various  appendages. 

Stage  E {—  Kingsley,  85’,  Fig.  5 ; Packard,  ’72,  Fig.  12).  — In 
this  stage  the  edge  of  the  carapax  has  been  differentiated,  form- 
ing a clear-cut  line  marking  off  the  ventral  from  the  dorsal  sur- 
face. The  six  pairs  of  cephalothoracic  legs  retain  a post-oral 
position,  while  the  first  pair  (operculum)  of  abdominal  appen- 
dages is  outlined.^ 

Stage  F (Packard,  Fig.  12;  Self,  '85,  Fig.  6;  present  article, 
Fig.  28).  — In  this  stage  the  embryo  is  much  as  before,  except 
that  the  second  (first  gill)  appendage  of  the  abdomen  has  made 
its  appearance,  while  the  series  of  sense  (.?)  organs  briefly  men- 
tioned by  Patten  (’89,  p.  602)  are  prominent,  especially  in  mounts 
peeled  from  the  egg  and  in  osmic  acid  preparations.  These 
sense  organs,  to  which  I shall  return  later,  are  six  in  number  on 
either  side  of  the  body.  I earlier  (’90)  described  their  fates,  which 
are  as  follows  : The  first  pair  give  rise  to  the  median  ocelli  of 
the  adult ; the  second  move  to  a position  in  front  of  the  mouth, 
where  near  the  median  line  they  form  a peculiar  sense  organ 
as  yet  undescribed ; the  third  and  sixth  disappear  at  a very 


1 This  is  not  well  shown  in  Packard’s  figures. 


52 


KINGSLEY. 


[VOL.  VII. 


early  day ; the  fourth  forms  the  structure  called  by  Watase 
(’89  and  ’90^)  the  “dorsal  organ,”  which  early  reaches  a large 
size  and  then  disappears ; while  I believe  that  the  fifth  gives 
rise  to  the  compound  eye.^  I now  believe  that  this  account 
will  require  serious  modification.  Of  the  existence  of  the  organs 
there  is  no  doubt,  but  their  fate  is  in  question. 

Stage  G (Fig.  32)  is  characterized  by  the  relative  change  in 
position  of  mouth  and  the  first  pair  of  limbs.  At  first  the 
mouth  is  distinctly  pre-appendicular  {vide  Figs.  27,  28,  29).  At 
this  time  its  shape  is  approximately  circular.  Soon,  however, 
the  mouth  becomes  more  elongate,  its  front  margin  becoming 
acute  as  if  the  right  and  left  lips  were  coalescing  (Figs.  30,  31). 
By  this  process  a true  ectodermal  stomodaeum  is  invaginated, 
and  the  mouth  is  carried  backward,  as  I have  already  explained 
and  diagrammatically  illustrated  (’85,  PL  XXXIX,  p.  526,  Figs. 
40-43),  so  that  as  a result  the  first  pair  of  appendages  become 
distinctly  post-oral.  Other  features  are  the  budding  of  . the 
curious  appendix  (flabellum  Auct.;  appendice  lanceole  de  la 
hanche,  van  der  Hoeven)  upon  the  basal  joint  of  the  sixth  pair 
of  appendages ; and  the  outlining  of  the  so-called  metastoma 
upon  the  sixth  body  segment.  I have  already  pointed  out  that 
this  last  cannot  be  regarded  as  an  appendage  of  a metameric 
nature  (Self,  ’85,  p.  532),  since  it  is  borne  on  the  same  segment 
as  the  true  sixth  appendage. 

In  Stage  Hy  Fig.  33  (Packard,  Fig.  19;  Kingsley,  ’85,  Fig.  12), 
the  distinction  between  cephalothorax  and  abdomen  is  evident ; 
the  legs  are  longer  and  show  evident  segmentation.  (Fig.  33.) 

In  Stage  I (Kingsley,  ’85,  Fig.  14 ; Packard,  Fig.  24)  the 
appearance  is  quite  like  that  of  the  adult.  The  body  is  now 
much  more  depressed,  the  legs  are  like  those  of  the  adult,  and 
the  cephalothorax  is  considerably  larger  than  the  abdomen. 
The  abdomen  exhibits  traces  of  segmentation,  while  its  margin 
bears  the  movable  spines  upon  its  margin  which  are  character- 
istic of  the  adult.  The  telson  as  yet  remains  as  a slight  lobe  of 
the  middle  of  the  hinder  margin  of  the  abdomen. 

1 This  account  varies  from  that  of  Patten,  (’90)  if  I understand  him  correctly. 
According  to  him  the  median  eye  falls  outside  the  category  of  these  organs.  The 
compound  eye  (“  convex  eye  ”)  “ arises  from  three  small  sense  organs  near  the  third 
thoracic  segment,”  while  the  “ eye  of  the  fourth  segment  ” is  very  large,  thus  putting 
the  compound  eye  in  front  of  the  ‘ dorsal  organ.’  Watase,  on  the  other  hand  (’90^) 
places  the  compound  eye  behind  the  dorsal  organ.  (See  Postscript.) 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


53 


Stage  Ky  Figs.  34  and  35  (Packard,  Fig.  25  ; Self,  Figs.  16  and 
17)  is  the  last  stage  previous  to  the  molt  which  results  in  the 
adult  form.  The  abdomen  is  relatively  much  larger  than  before ; 
the  opercular  lobes  have  nearly  met  in  the  median  line,  and  the 
animal  begins  to  burrow  in  the  sand,  although  embryos  of  this 
stage  are  not  infrequently  taken  in  the  towing  net. 

Stage  L (Packard,  Fig.  27)  is  produced  from  the  last  by  a 
single  molt.  It  is  characterized  by  the  presence  of  an  elongate 
telson  much  like  that  of  the  adult.  With  this  stage  my  studies 
end. 

The  following  points  may  also  be  of  interest.  The  Blasto- 
dermhaut  is  molted  at  about  Stage  F,  the  time  varying  with 
different  eggs.  It  still  persists  as  an  embryonic  envelope 
(vicarious  chorion  of  Packard)  until  a late  stage.  Soon  after  it 
is  shed  from  the  parent  cells  a second  embryonic  cuticle  is  cast, 
and  then  the  true  chorion  is  shed,  and  the  embryo,  encased  in 
the  distended  Blastodermhaut,  escapes  from  the  egg  at  about 
Stage  K or  L.  The  Blastodermhaut  itself  is  ruptured,  and  the 
animal  begins  its  free  existence  at  the  end  of  Stage  I. 

Comparisons. 

A.  With  Previous  Accounts.  — H.  L.  Osborn  (’85)  and  Brooks 
and  Bruce  (’85)  have  described  some  of  the  phases  of  segmen- 
tation, the  latter  studying  sections.  Their  account  so  far  as  it 
goes  is  reconcilable  with  what  I have  described,  including  the 
pre-segmental  movements.  They  have  also  noticed  the  primitive 
cumulus  and  interpret  it  as  giving  rise  to  the  mesoderm,  a point 
to  be  discussed  later.  Neither,  however,  traces  the  relationship 
of  the  cumulus  to  the  embryo.  According  to  the  last  quoted 
paper  the  blastoderm  is  to  be  regarded  as  ecto-mesoderm,  the 
yolk  as  at  least  largely,  if  not  wholly,  entoderm. 

Packard  (’72)  has  apparently  seen  some  of  the  phases  of  seg- 
mentation, but  it  is  difficult  to  arrange  his  account  in  its  proper 
order,  as  it  is  evident  that  some  of  his  eggs  were  addled.  In 
others  he  figures  nuclei  which  had  no  actual  existence.  From 
segmentation  until  the  appearance  of  the  limbs  Packard  has  seen 
nothing  except  the  formation  of  the  Blastodermhaut,  which  he 
in  various  papers  has  compared  to  the  Hexapod  amnion  — a view 
which  I (’84)  showed  to  be  untenable.  H.  L.  Osborn’s  account 
of  the  formation  of  the  limbs,  etc.,  I have  referred  to  above 


54 


KINGSLEY. 


[VOL.  VII. 


(p.  50).  Patten  has  incidentally  described  some  of  the  early 
stages  of  Limulus  (’90).  Packard,  Dohrn,  Lockwood,  et  al.  have 
described  the  later  stages,  and  the  foregoing  brief  r^sum^  calls 
for  no  comparisons  with  their  results.  (See  Postscript.) 

B.  With  Other  Arthropods.  — Three  types  of  segmentation 
of  the  egg  may  be  recognized  in  the  Arthropods. 

In  the  first,  examples  of  which  are  furnished  by  the  lower 
Crustacea,  Lucifer,  (.?)  Palaemon  (Bobretzky),  Phronima,  Chelifer, 
Gammarus  locusta  (Van  Beneden  and  Bessels),  Pycnogonids 
(Morgan),  etc.,  the  egg  undergoes  a regular  or  irregular  total 
segmentation  (holoblastic). 

In  the  second  the  egg  consists  of  a central  nucleus  and 
protoplasm  with  peripheral  yolk.  The  central  protoplasm  seg- 
ments, but  until  several  or  many  blastomeres  result,  the  yolk 
remains  undivided.  This  is  the  type  usually  called  cen- 
trolecithal,  or  endolecithal  (Claus)  and  superficial.  I have 
already  pointed  out  with  some  detail  (’86,  pp.  112-138)  that 
these  terms  are  misleading,  and  would  substitute  ectolecithal 
therefor.  ‘ Superficial  segmentation  ’ as  usually  described  is 
characteristic  only  of  late  stages  of  ectolecithal  or  of  meroblastic 
eggs.  In  these  ectolecithal  eggs  two  secondary  modifications 
are  noticeable.  In  the  one  the  yolk  is  extracellular;  it  lies 
between  the  cells  formed  by  the  dividing  protoplasm  and 
nuclei,  as  in  Phryganids  (Patten,  ’85),  Crangon  (Kingsley, 
’86),  and  Julus  (Heathcote,  ’86).  In  the  other  the  yolk  itself 
becomes  divided,  forming  balls  (true  yolk  cells),  in  the  centre 
of  each  of  which  the  nucleus  and  protoplasm  occur  (examples, 
most  Hexapods).^  Of  these  the  second  is  structurally,  if  not 
phylogenetically,  nearest  to  the  meroblastic  type. 

In  the  third  or  meroblastic  type  the  segmentation  is,  strictly 
speaking,  superficial,  and  is  at  first  confined  to  one  side  of  the 
egg.  Instances  are  less  common  among  the  Arthropods  than 
of  the  other  two,  although  several  have  been  described;  e.g. 
Scorpio  (Metschnikoff,  ’71 ; Laurie,  ’90),  Mysis  (Van  Beneden), 
Oniscus^  (Bobretzky). 

1 Mereschowski  (’82)  has  described  what  he  regards  as  a fourth  type,  occurring  in 
Callianassa  mediterranea.  It  is  plainly  closely  related  to  the  second  modification 
just  mentioned. 

2 According  to  Reinhard’s  brief  note  (’87)  it  would  appear  as  if  in  Porcellio 
the  segmentation  was  of  the  ectolecithal  type,  and  that  the  meroblastic  conditions 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


55 


Owing  to  my  inability  to  find  the  segmentation  nucleus,  I am 
unable  to  say  with  certainty  to  which  of  the  types  the  egg  of 
Limulus  should  be  referred,  but  all  the  facts  point  towards  the 
second  modification  of  the  ectolecithal  type.  However,  seg- 
mentation is  at  best  an  uncertain  guide  to  affinities. 

The  matter  of  differentiation  of  the  germ  layers  is  more 
important.  Until  recently  delamination  was  believed  to  be 
confined  to  the  Coelenterates  and  a few  other  forms. ^ It  would 
appear,  however,  that  delamination  is  of  frequent  occurrence  in 
the  Arachnid  phylum.  Morgan  finds  in  the  Pycnogonids  (’90) 
a true  multipolar  delamination,  and  he  uses  this  as  one  reason 
for  assigning  these  forms  to  a position  near  the  Arachnids. 
He  refers  to  Chelifer  as  described  by  Metschnikoff  and  to 
Balfour’s  account  of  Agelena,  and  to  these  additional  references 
may  be  given.  Locy  (’86,  pp.  74-75)  clearly  confirms  Balfour 
so  far  as  Agelena  is  concerned ; Henking  (’86)  describes  a 
delaminate  type  of  blastoderm  formation  in  the  Phalangids, 
while  Faussek  (’9l),  studying  the  same  forms,  is  in  full  accord 
and  expressly  uses  the  term  delamination  in  this  connection. 
Schimkewitch  (’84  and  ’87)  also  clearly  describes  delamination 
in  Epeira,  Pholcus,  Agelena,  and  Lycosa. 

On  the  other  hand,  the  following  forms  have  the  yolk  at  one 
time  free  from  nuclei,  and  hence,  if  delamination  occur  in  con- 
nection with  the  primitive  keel,  it  is  not  of  that  type  which 
obtains  in  the  cases  mentioned  above : Theridion  (Morin, 
’87)  at  the  128-cell  stage  ; a Japanese  species  of  Agelena  (Kishe- 
nouye,  ’90),  Scorpion  (Kowalewsky  and  Schulgin,  ’86;  Laurie,  ’90). 

So  far  as  I know,  nothing  approaching  delamination  occurs  in 
the  Crustacea,  while  that  in  the  Tracheates,  already  referred  to, 
is  of  a character  far  different  from  that  in  the  Arachnids. 
Hence  Limulus,  in  the  method  of  differentiation  of  entoderm 
from  ecto-mesoderm,  finds  its  closest  analogues  within  the 
Arachnid  phylum. 

resulted  from  a migration  of  the  blastomeres  to  one  pole  of  the  egg.  Dr.  McMurrich 
informs  me  that,  according  to  his  observations  on  both  Porcellio  and  Armidillidium, 
the  segmentation  is  as  I have  interpreted  it  in  this  note,  — a fact  which  would  tend 
to  show  that  Bobretzky  described  a stage  too  late  to  decide  the  question. 

1 Balfour  (’81),  p.  278,  compares  the  origin  of  the  germ  layers  in  most  ‘Trach- 
eates ’ to  a type  which  approaches  delamination,  but  he  expressly  states  that  there 
are  strong  grounds  for  regarding  it  as  “ a secondary  modification  of  an  invaginate 
type.” 


56 


KINGSLEY. 


[VoL.  VIL 


There  can  be  no  question  that  delamination  in  these  forms  is 
not  a direct  derivative  from  delamination  in  the  Coelenterates. 
It  has  rather  arisen  in  the  Arachnids  and  probably  from  a true 
gastrulate  type.  The  considerations  which  lead  to  this  con- 
clusion are  these: — 

It  is  at  least  probable  that  the  Arthropods  have  had  an  anne- 
lidan  ancestry,  and  in  these  latter  forms  delamination  does  not 
occur.  Hence  we  must  either  regard  it  as  having  been  lost  in 
the  segmented  worms  - while  it  is  retained  in  the  Arachnids,  or 
we  must  consider  it  as  of  csenogenetic  character  in  the  latter 
group.  I believe  that  delamination,  as  it  occurs  in  Limulus  and 
the  Pycnogonids,  may  be  traced  back  to  an  ancestral  invaginate 
condition  ; in  fact,  all  stages  between  a regular  embolic  gastrula 
like  that  of  Lucifer  and  the  extreme  delamination  of  the 
Pycnogonids  can  be  found  in  the  Arthropod  phylum,  although 
not  in  the  Arachnids  themselves. 

The  series  between  Lucifer  (Brooks,  ’82)  with  an  arch- 
enteric cavity  of  large  size  is  easily  traced  through  conditions 
like  those  of  Astacus  and  Palaemon,  to  that  presented  by  Cran- 
gon,  where  the  invaginated  entoderm  is  solid,  but  in  which  the 
blastopore  is  still  readily  recognized.  Crangon,  on  the  other 
hand,  presents  many  similarities  to  Theridion  (Morin,  ’87)  and 
the  Japanese  species  of  Agelena  studied  by  Kishenouye.  In  the 
forms  just  mentioned  there  is  apparently^  a time  when  every 
nucleus  has  reached  the  surface  and  has  participated  in  the  for- 
mation of  the  blastoderm,  leaving  the  large  central  yolk  in  an 
anneliate  condition.  Later,  the  blastoderm  thus  formed  becomes 
thickened  by  cell  proliferation,  and  from  the  ridge  thus  formed 
cells  pass  “ into  the  yolk  and  become  scattered  without  definite 
arrangement  through  the  entire  yolk.  These  are  the  entoderm 
cells”  (Kishenouye,  p.  62  ; cf.  Kingsley,  ’86,  p.  no). 

Now  in  forms  like  Astacus,  Palaemon,  and  Crangon  the  meso- 
derm arises  from  the  lips  of  the  blastopore  and  from  what  may 
be  regarded  as  its  forward  continuation  in  the  median  line,  and 
from  this  fact  we  are  justified  in  regarding  the  thickening  which 
in  the  Japanese  Agelena  and  in  Scorpio  (Laurie)  gives  rise  to 
mesoderm  and  entoderm  as  an  obsolescent  blastopore  homologous 
with  the  actual  open  blastopore  in  the  other  forms  mentioned. 

1 Kishenouye  could  not  “ detect  any  nucleus  at  all  in  the  yolk,  thus  confirming 
the  views  of  Morin  in  opposition  to  Balfour’s”  (’90,  p.  60). 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


57 


The  transition  from  the  Japanese  Agelena  and  Scorpio  to  a 
true  delamination  is  greater  than  that  already  traced  ; and  as  yet, 
so  far  as  the  literature  at  hand  enables  me  to  decide,  it  cannot 
be  traced  without  going  outside  the  limited  group  of  Arachnids. 
Still  the  successive  stages  are  readily  imagined. 

In  the  ectolecithal  egg  the  blastoderm  arises  by  migration  of 
the  primitively  central  cells  to  the  periphery,'  and  in  many 
forms  every  nucleus  goes  through  this  migration,  leaving  the 
yolk  at  one  period  entirely  free  from  cells.  In  others  only  a 
portion  of  the  cells  reach  the  surface,  the  others  remaining 
behind  in  the  yolk.  Concerning  the  fate  of  these  latter,  opinions 
differ.  In  some  forms  they  are  described  as  playing  no  part  in 
the  building  up  of  the  embryo,  but  rather  acting  as  ‘vitellophags,’ 
the  sole  function  of  which  is  to  gradually  metabolize  the  deuto- 
plasm, after  which  they  disappear.  On  the  other  hand,  in- 
stances are  not  wanting  in  which  these  yolk  cells  are  to  be 
regarded  as  true  entoderm  cells,  from  which  later  the  epithelium 
of  the  mid-gut  is  to  be  built  up.  This  is  the  case  with  Limulus, 
as  I shall  detail  later,  and  apparently  also  in  many  Araneina  and 
Hexapods. 

With  such  conditions  as  are  afforded  by  Crangon,  Theridion, 
etc.,  it  can  readily  be  seen  that  any  acceleration  of  develop- 
ment which  would  prevent  certain  of  the  central  blastomeres 
from  migrating  to  the  surface,  only  to  be  immediately  returned 
as  entoderm,  would  be  a distinct  gain ; and  this,  in  my  opinion, 
is  the  way  the  peculiar  conditions  in  many  Hexapods  have  been 
brought  about.  At  least,  this  view  has  the  merit  of  rendering 
intelligible  many  features  of  Arthropod  ontogeny  which  other- 
wise are  not  readily  understood. 

A farther  step  in  the  same  direction  is  afforded  by  Limulus,, 
where  a farther  economy  is  seen  in  the  cutting  off  of  the  periph- 
eral from  the  deeper  ends  of  the  cells,  thus  at  once  differen- 
tiating an  outer  ecto-mesodermal  layer  from  an  inner  entoderm 
rich  in  food  yolk.  The  final  stage,  as  we  know  it,  is  seen  in 
Tanystylum  and  Phoxichilidium  as  described  by  Morgan  (’90). 
Here  the  egg  is  much  reduced  in  size,  the  blastomeres  are 
fewer,  and  each  cell  is  at  once  (apparently)  differentiated  into 
entodermal  and  ecto-mesodermal  portions,  the  result  being  a 
condition  which  closely  simulates  the  multipolar  delamination 
found  in  Geryonia,  made  classic  by  the  researches  of  Fol  and 


58 


KINGSLEY. 


[VOL.  VII. 


Metschnikoff,  but  of  course  without  actual  phyletic  connection 
with  it. 

Our  knowledge  of  mesoderm  development  in  the  Arthropods 
is  far  from  complete,  and  at  present  it  is  not  possible  to  point 
out  the  peculiarities  which  characterize  the  different  groups. 
My  account  of  mesoderm  formation,  as  it  occurs  in  Limulus, 
agrees  well  in  its  major  features  with  the  account  of  Patten 
(’90),  except  that  he  describes  at  the  posterior  end  of  the  em- 
bryo a slit-like  ” primitive  streak,  and  he  further  regards  the 
proliferated  cells  as  both  mesoderm  and  entoderm  (p.  373). 
The  lateral  connection  of  mesoderm  and  ectoderm  he  compares 
with  the  Keimwall  of  the  Vertebrates — a point  upon  which  I 
would  rather  admit  analogy  than  actual  homology. 

The  accounts  of  mesoderm  formation  in  Scorpio  differ. 
Laurie  (’90)  describes  the  inpushing  of  a mes-entoderm  from  all 
parts  of  the  upper  (outer)  surface  of  which  the  mesoderm  is 
afterward  proliferated.  Patten  (’90),  on  the  other  hand,  de- 
scribes a median  posterior  thickening  from  which  cells  grow  for- 
ward and  laterally,  the  median  portion  forming  the  sexual  organs 
and  botryoidal  cord  ; the  lateral,  the  mesoderm  and  entoderm. 

In  the  Decapodous  and  Isopodous  Crustacea  the  mesoderm 
would  appear  to  grow  forward  as  two  bands  from  the  anterior 
margin  and  sides  of  the  blastopore.  In  some  Cladocera  and 
Copepods  (Grobben,  ’79  and  ’8I)  somewhat  similar  conditions 
may  be  traced,  except  that  the  primitive  mesoderm  cells  are 
behind  the  point  of  entodermal  invagination.  In  Cyclops,  on 
the  other  hand  (Urbanowicz,  ’84),  mesenchyme  is  described  as 
budding  from  the  blastoderm  cells,  and  Ulianin  (’8i)  describes 
the  same  in  Orchestia. 

In  the  Arachnids  our  knowledge  of  mesoderm  formation  is 
extremely  scanty.  All  agree,  so  far  as  the  Araneida  are  con- 
cerned, that  the  primitive  cumulus  and  posterior  cloud  are  con- 
cerned in  the  process,  and  some  show  that  at  first  the  mesoderm 
forms  a continuous  band  across  the  embryo.  A comparison 
of  figures  {e.g.  Locy,  86,  Fig.  49)  of  Arachnid  embryos  with 
my  own  of  Limulus  will,  I think,  show  similarities  which  cannot 
be  paralleled  by  similar  resemblances  between  Limulus  and  the 
Crustacea. 

In  the  differentiation  of  the  germ  the  resemblances  of  Limu- 
lus to  the  Arachnids  are  striking.  So  far  as  I know  primitive 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


59 


cumulus  and  posterior  cloud  occur  only  in  these  forms ; and  the 
succeeding  stages  are  almost  equally  close.  As  I correllate 
them,  my  figures  of  Limulus  are  to  be  compared  with  those  of 
the  true  Arachnids  as  follows  : — 


Limulus.  Arachnid  a. 

Fig.  21 Agelena,  Locy,  Fig.  i ; Kishenouye,  Fig.  5. 

Fig.  23 Agelena,  Kishenouye,  Fig.  5 ; Balfour,  Fig.  i. 

Fig.  24  .....  . Agelena,  Locy,  Fig.  3 ; Scorpio,  Metschnikoff,  PI. 

XVII,  Fig.  2. 

Fig.  25 Scorpio,  Metschnikoff,  PI.  XVII,  Fig.  3 (one  less 

somite)  ; Laurie,  Fig.  17  (one  more  segment 
and  lacks  primitive  groove) . 

Fig.  26  ......  Agelena,  Schimkewitsch,  PI.  XVIII,  Fig.  I ; Balfour, 

Fig.  3.  Locy,  Fig.  6;  Scorpio,  Metschnikoff, 
PI.  XVII,  Fig.  6. 


A slight  comparison  of  these  figures  will  show  that  previous 
to  the  appearance  of  the  limbs  there  are  a remarkable  series  of 
parallels.  Limulus  agrees  with  the  Arachnids  and  differs 
from  the  Crustacea  in  the  external  appearance  and  growth 
of  the  germinal  disc ; in  the  considerable  development  of 
metamerism  before  the  appearance  of  the  appendages,^  and  in 
the  simultaneous  appearance  of  the  anterior  five  or  six  pairs  of 
appendages.  When  one  of  the  six  is  lacking  at  first,  it  is 
apparently  the  anterior  pair  which  forms  later.  This  has  been 
shown  by  Balfour,  Schimkewitsch,  and  Kishenouye  in  Agelena ; 
by  Metschnikoff  and  Laurie  in  Scorpio,  and  by  Dohrn  and 
myself  in  the  present  paper.  On  the  other  hand,  Claparede 
(’68)  describes  the  sixth  pair  as  lacking  in  Myobia,  and  Van 
Beneden  (’51)  gives  the  same  account  of  Atax.  Limulus  agrees 
with  the  Arachnids  and  differs  from  the  Crustacea  in  the  total 
absence  of  a nauplius  stage. 

August,  1891. 


POSTSCRIPT. 

Since  the  foregoing  pages  were  in  the  printer’s  hands  K.  Kish- 
enouye has  published  his  complete  paper  on  the  development 

^ In  Chelifer  (Metschnikoff,  ’70),  the  chelicerse  apparently  are  formed  before 
the  somites  are  outlined. 


6o 


KINGSLEY. 


[VOL.  VII. 


of  the  Japanese  King  Crab  {L.  longispina),  which  presents  some 
points  of  difference  from  the  L.  polyphemus  of  the  Atlantic 
coast.  Some  of  these  variations  may  be  noticed  here. 

In  the  external  development  Kishenouye  did  not  distinguish 
between  primitive  cumulus  and  posterior  cloud.  In  the  process 
of  metamerism  the  first  line  of  demarcation  occurs  between 
somites  I and  II,  while  the  appearance  which  I have  called  the 
primitive  streak  does  not  occur  until  two  somites  are  differen- 
tiated from  the  anterior  and  posterior  areas.  In  the  later  stages 
he  finds  organs  homologous  with  the  flabellum  of  appendage  VI, 
occurring  as  transitory  rudiments  on  somites  2-5.  These  are 
clearly  not  homologous  with  the  peculiar  (sense  .?)  organs  men- 
tioned on  p.  49,  since  the  latter  occur  outside  the  ventral  disc, 
while  the  flabella  of  Kishenouye  are  all  within  that  area. 

In  the  internal  development  the  discrepancies  are  more  im- 
portant. Thus  Kishenouye  describes  the  ectoderm  as  separat- 
ing from  lower-layer  cells,  and  says  that  the  mesoderm  has 
three  origins : (i)  from  the  lower-layer  cells,  (2)  from  the 
edges  of  the  primitive  streak,  which  is  confined  to  the  posterior 
portion  of  the  ventral  disc,  and  (3)  from  cells  in  the  dorsal 
region  which  migrate  from  the  yolk.  The  primitive  streak 
mesoderm  is  confined  to  the  abdominal  region,  while  that 
derived  from  the  lower-layer  cells  gives  rise  to  the  tissues  of 
the  cephalothorax. 

A still  farther  point  of  difference  is  with  regard  to  the  meta- 
stoma. This  Kishenouye  regards  as  a true  appendage  serially 
homologous  with  the  other  appendage  of  the  body.  In  this 
I cannot  agree  with  him.  My  observations  show  no  metastomal 
somite  and  no  corresponding  neuromere.^  On  the  other  hand, 
it  seems  probable  that  there  is  here  an  error  in  interpretation, 
for  a study  of  his  figures  inclines  me  to  believe  that  his  meta- 
stoma is  in  reality  the  operculum,  and  that  the  following  appen- 
dages are  to  be  correspondingly  changed.  The  other  points  of 
difference  will  be  discussed  in  the  second  part  of  this  paper. 

Tufts  College,  Mass.,  August,  1892. 


1 See  Kingsley,  ’85,  p.  532,  PI.  XXXVIII,  Fig.  22. 


No.  I.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


6l 


LITERATURE. 

’78  Agassiz,  A.  Note  on  the  Habits  of  the  Young  Limulus.  Am.  Jour. 
Sci.,  Ill,  XV,  pp.  75-76.  1878. 

’80  Balfour,  F.  M.  Notes  on  the  Development  of  the  Araneina.  Quarterly 
Jour.  Micros.  Sci.,  xx,  pp.  167-189;  PI.  xix-xxi.  1880. 

’81  Balfour,  F.  M.  A Treatise  on  Comparative  Embryology.  Vol.  I. 
London,  1881. 

’83  Benham,  W.  B.  S.  On  the  Testis  of  Limulus.  Trans.  Linn.  Socy.  II, 
Zool.,  ii,  pp.  363-366;  PI.  38.  1883. 

’73  Bobretzky,  N.  In  Me7n.  Kiew  Nat.  Soc.,  hi,  pp.  129-263;  Pis.  i-vi. 
1873.  (I  have  used  only  Hoyer’s  Abstract  in  Hoffmann  & Schwalbe’s 
Jahresberichte,  ii,  p.  312.  1875.) 

’74  Bobretzky,  N.  Zur  Embryologie  des  Oniscus  murarius.  Zeit.  wiss. 

Zool.,  xxiv,  pp.  179-203;  Pis.  21-22.  1874. 

’82  Brooks,  W.K.  Lucifer : a Study  in  Morphology.  Phil.  Tra7is.,  Qclxxm, 
p.  57.  1882. 

’85  Brooks,  W.  K.,  and  Bruce,  A.  T.  Abstract  of  Researches  on  the  Em- 
bryology of  Limulus  polyphemus.  Johns  Hopkins  Univ.  Circ.,  v,  pp. 
2-4.  1885. 

’62  Claparede,  E.  Recherches  sur  I’E volution  des  Araignees.  Natuurk. 

Verhandl.  Utrechts  Genootsch.,  D.  i,  S.  i,  pp.  92  ; Pis.  8.  1862. 

’68  Claparede,  E.  Studien  an  Acariden.  Zeit.  wiss.  Zool.,  xviii,  pp.  445- 
546;  Pis.  xxx-xl.  1868. 

’71  Dohrn,  a.  Zur  Embryologie  und  Morphologie  des  Limulus  polyphemus. 
Jena.  Zeitsch.,  vi.  1871. 

’90  Eyclesheimer,  A.  C.  Celloidin  Imbedding  in  Plant  Histology.  Botan. 
Gazette,  xv,  p.  292.  1890. 

’91  Faussek,  V.  Zur  Embryologie  von  Phalangium.  Zool.  Anz.,Y\v,^.  3. 
1891. 

’82  Faxon,  W.  Selections  from  Embryological  Monographs.  I.  Crusta- 
cea. Memoirs  Mus.  Comp.  Zool.,  ix.  1882. 

’58  Gegenbaur,  C.  Anatomische  Untersuchungen  eines  Limulus.  Abh. 
Naturf.  Gesellsch.  Halle,  iv.  1858. 

’79  Grobben,  C.  Die  Entwickelungsgeschichte  des  Moina.  Arb.  zool. 
Inst.  Wien,  ii.  1879. 

’81  Grobben,  C.  Die  Entwickelungsgeschichte  von  Cetochilus.  Arb.  zool. 
Inst.  Wien,  iii.  1881. 

’86  Heathcote,  F.  G.  The  Early  Development  of  Julus  terrestris.  Qicart. 
Jour.  M.  S.,  xxvi.  1886. 

’86  Henking,  H.  Untersuchungen  iiber  die  Entwicklung  der  Phalangiden. 
Zeit.  wiss.  Zool.,  xlv.  1886. 

’89  Heidlr,  Karl.  Die  Embryonalentwicklung  von  Hydrophilus  piceus. 
Jena,  1889. 

’85  Howell,  W.  H.  Observations  upon  the  Chemical  Composition  and 
Coagulation  of  the  Blood  of  Limulus  polyphemus,  Callinectes  hastatus, 
and  Cucumaria  sp.  Johns  Hopkins  Circ.,  v,  p.  4.  1885. 


62 


KINGSLEY. 


[VoL.  VIL 


’84  Kingsley,  J.  S.  The  Development  of  Limulus.  Science  Record,  ii, 
p.  249.  1884. 

’85  Kingsley,  J.  S.  Notes  on  the  Embryology  of  Limulus.  Quar.  Jour. 
Mic.  Sci.,  XXV.  1885. 

’86  Kingsley,  J.  S.  The  Development  of  Crangon  vulgaris.  Bulletin 
Essex  Ifist.y -Kviii.  1886. 

’90  Kingsley,  J.  S.  The  Ontogeny  of  Limulus.  Amer.  Nat.,  xxiv,  p.  678. 

1890. — Zool.  Anz.,  xiii,  p.  536.  1890. 

’90  Kishinouye,  K.  On  the  Development  of  the  Araneina.  Jour.  Coll. 
Sci.  Bnp.  Univ.  Japan^,  iv.  1890. 

’86  Kowalevsky,  a.  and  Schulgin,  M.  Zur  Entwicklungsgeschichte  des 
Scorpions.  Biol.  Centralbl.  vi.  1886. 

’78  Lankester,  E.  R.  Motility  of  Spermatozoids  of  Limulus.  Quar.  Jour. 
Mic.  Sci.,  xviii.  1878. 

’90  Laurie,  M.  The  Embryology  of  a Scorpion  (Euscorpius  italicus). 
Qr.  J.  M.  S.,  xxxi.  1890. 

’70  Lockwood,  S.  The  Horse-foot  Crab.  Am.  Nat.,  iv.  1870. 

’86  Locy,  W.  a.  Observations  on  the  Development  of  Agelena  naevia. 
Bull.  M.  C.  Z.,  xii.  1886. 

’75  Ludwig,  H.  Ueber  Eibildung  im  Thierreiche.  Arb.  z.  z.  Inst.  Wurz- 
burg, i.  1875. 

’77  Mayer,  P.  Zur  Entwicklungsgeschichte  der  Decapoden.  Jena.  Zeit., 
xi.  1877. 

’82  Mereschkowski,  C.  V.  Eine  neue  Art  von  Blastodermbildung  bei  den 
Decapoden.  Zool.  Anz.,  v.  1882. 

’66  Metschnikoff,  E,  Embryologische  Studien  an  Insecten.  Zeit.  w. 
Zool.,  xvi.  1866. 

’68  Metschnikoff,  E.  Istoria  Razvitia  Nebalia.  Zap.  Imp.  Acad.  St. 
Peterb.,  xiii.  1868. 

’70  Metschnikoff,  E.  Entwicklungsgeschichte  des  Chelifer.  Z.  w.  Z., 
xxi.  1870. 

’71  Metschnikoff,  E.  Embry ologie  des  Scorpions.  Z.  w.  Z.,  xxi.  1871. 
’38  Milne-Ed WARDS,  H.  Recherches  Relatives  au  Ddveloppement  des  Lim- 
ulus. Ext.  Proc.  Verb.  Soc.  Philomath.  Paris.  1838.  (Teste  van  der 
Hoeven.) 

’39  Milne-Edwards,  H.  In  Regne  Animal  de  Cuvier,  edition  illustre. 

Crustaces,  PI.  76,  Figs.  2 1 2 i.  “ 1839.” 

’40  Milne-Edwards,  H.  Histoire  Naturelle  des  Crustacds,  tom  iii,  pp.  538- 
551.  Paris,  1840. 

’91  Morgan,  T.  H.  A Contribution  to  the  Embryology  and  Phylogeny  of 
the  Pycnogonids.  Studies  Biol.  Lab.  J.  H.  Univ.,  v.  1891. 

’87  Morin,  I.  Zur  Entwicklungsgeschichte  der  Spinnen.  Biol.  Cbl.,  vi. 
1887. 

’85  Osborn,  H.  L.  The  Metamorphosis  of  Limulus  polyphemus.  Johns 
Hopkins  Circ.,  v.  1885. 

’72  Owen,  R.  On  the  Anatomy  of  the  American  King  Crab.  Trans.  Linn. 
Soc.,  Zool.  xxviii.  1872. 

’70^  Packard,  A.  S.  The  Embryology  of  Limulus  polyphemus.  Am. 
Nat.,  iv,  p.  498.  1870. 


THE  EMBRYOLOGY  OF  LIMULUS. 


No.  I.] 


63 


’70*^  Packard,  A.  S.  Morphology  and  Ancestry  of  the  King  Crab.  Ajn. 
Nat.,  iv,  p.  754.  1870. 

’70*^  Packard,  A.  S.  An  Account  of  the  Development  of  Limulus  poly- 
phemus.  Proc.  Post.  Soc.  Nat.  Hist.,  xiv.  1870. 

’71  Packard,  A.  S.  On  the  Embryology  of  Limulus  polyphemus.  Proc. 
Am.  Assoc.  Adv.  Sci.,  xix.  1871.  — Quar.  Jour.  Mic.  Set.,  xi. 
1871. 

’72  Packard,  A.  S.  The  Development  of  Limulus  polyphemus.  Mem. 
Bost.  Soc.  N.  H.,  ii.  1872. 

’73  Packard,  A.  S.  Farther  Observations  on  the  Embryology  of  Limulus, 
with  Notes  on  its  Affinities.  Am.  Nat.,  vii,  p.  675.  1873. 

’75  Packard,  A.  S.  On  the  Development  of  the  Nervous  System  of 
Limulus.  Am.  Nat.,  ix,  p.  422.  1875. 

’80  Packard,  A.  S.  The  Anatomy,  Histology,  and  Embryology  of  Limulus 
polyphemus.  Anniv.  Mem.  Bost.  Soc.  N.  H.,  1880. 

’85  On  the  Embryology  of  Limulus  polyphemus.  III.  Proc.  Am.  Phil.  Soc., 
xxii.  1885.  — Am.  Nat.,  xix,  p.  722.  1885. 

’60-61  Pagenstecher,  H.  A.  Beitrage  zur  Anatomie  der  Milben.  Leip- 
zig, 1860-61. 

’84  Patten,  W.  The  Development  of  Phryganids,  with  a preliminary  note 
on  the  development  of  Blatta  germanica.  Q.  J.  M.  S.,  xxiv,  1884. 
’89  Patten,  W.  The  Segmental  Sense  Organs  of  Arthropods.  Jotir. 
Morph.,  ii.  1889. 

’90  Patten,  W.  On  the  Origin  of  Vertebrates  from  Arachnids.  Q.  J. 
M.  S.,  xxxi.  1890. 

’86  Reichenbach,  H.  Studien  zur  Entwicklungsgeschichte  des  Fluss- 
krebses.  Abh.  Senckenh.  Nat.  Gesell.,  xiv.  1886. 

’87  Reinhard,  W.  Zur  Ontogenie  des  Porcellio  scaber.  Zool.  Anz.,  x, 
p.  9.  1887. 

’87  SCHIMKEWITSCH,  W.  Etude  sur  le  Developpement  des  Araignees. 
Arch,  de  Biol.,  vi.  1887. 

’86  Stuhlmann,  F.  Die  Reifung  des  Arthropodeneies  nach  Beobachtungen 
an  Insekten,  Spinnen,  Myriapoden,  und  Peripatu.  Ber.  Naturf. 
Gesell.  z.  Freiburg  i.  B.,\.  1886. 

’81  Ulianin,  B.  Zur  Entwicklungsgeschichte  der  Amphipoden.  Z.  w.  Z., 

XXXV.  1881. 

’84  Urbanowicz,  F.  Zur  Entwicklungsgeschichte  der  Cyclopiden.  Zool. 
Anz.,  vii.  1884. 

’51  Van  Beneden,  P.  Developpement  de  I’Atax  ypsilophora.  Metn.  Acad. 
Roy.  Belg.,  xxiv.  1851. 

’69  Van  Beneden,  E.  Recherches  sur  I’Embryologie  des  Crustacds.  II. 

Ddveloppement  des  Mysis.  Btdl.  Acad.  Belg.,  II,  xxix.  1869. 

’38  VAN  DER  Hoeven,  J.  Recherclies  sur  I’Histoire  Naturelle  et  I’Anatomie 
des  Limulus.  Leyde,  1838. 

’89  VOELTZKOW,  A.  Entwicklung  im  Ei  von  Muscav  omitoria.  Arb.  z.  z. 
Inst.  Wurzburg,  ix.  1889. 

’89  Watase,  S.  On  the  Structure  and  Development  of  the  Eyes  of  Lim- 
ulus. Johns  Hopkins  Circ.,  viii,  p.  34.  1889. 


[VOL.  VII. 


64  KINGSLEY. 

’90®  Watase,  S.  On  the  Morphology  of  the  Compound  Eyes  of  Arthro- 
pods. Studies  Biol.  Lab.  Johns  Hopkins.,  iv.  1890. 

Watase,  S.  On  the  Migration  of  the  Retinal  Area  and  its  Relation  to 
the  Morphology  of  the  simple  (Ocelli)  and  compound  Eyes  of 
Arthropods.  Johns  Hopkins  Circ..,  x.  1890. 

’63  Weismann,  a.  Die  Entwicklung  der  Dipteren  im  Ei.  Z.  w.  Z.,  xiii. 
1863. 

’84  WiTLACZiL,  E.  Entwicklungsgeschichte  der  Aphiden.  Z.  w.  Z.,  xl. 
1884. 

’78  Whitman,  C.  O.  The  Embryology  of  Clepsine.  Q.  J.  M.  6’.,  xviii. 
1878. 


No.  I.] 


THE  EMBRYOLOGY  OE  LIMULUS 


65 


EXPLANATION  OF  THE  FIGURES. 
Reference  Letters. 


•ar. 

Artery. 

ov.  e. 

Ovarian  epithelium. 

bl. 

Blastema. 

pc. 

Primitive  cumulus. 

hs. 

Blood  sinus. 

Pg- 

Polar  globule  ? 

c. 

Cerebrum. 

pgr. 

Primitive  groove. 

ec. 

Ectoderm. 

po. 

Primordial  ovum. 

f 

Flabellum. 

pr. 

Protoplasmic  processes. 

Gill-bearing  appendage. 

ss. 

Segmental  structures  (glands  or 

/. 

Appendage  I. 

sense  organs?). 

1. 

Liver  tubule. 

X. 

Cell  in  process  of  delamination. 

me. 

Mesoderm. 

y- 

Yolk. 

mo. 

Mouth. 

z. 

Junction  of  ectoderm  and  meso- 

n. 

Neuromeres. 

derm  at  the  margin  of  the  ger- 

0. 

Ovum. 

minal  disc. 

op. 

Operculum. 

66 


KINGSLEY. 


DESCRIPTION  OF  PLATE  V. 

Figs,  i,  2.  Sections  (longitudinal  and  transverse)  through  a portion  of  the  liver 
and  ovary  of  a Limulus  four  inches  in  length,  showing  the  formation  of  the  primordial 
ova  and  the  intrusion  of  older  ova  between  the  ovarian  epithelium  and  tunica 
propria. 

Fig.  3.  Section  of  an  egg  one  hour  after  impregnation,  showing  a possible  polar 
globule. 

Figs.  4,  5,  6.  Surface  views  of  eggs  four  hours  after  impregnation,  showing  the 
peculiar  segmentation  of  the  surface  previous  to  true  segmentation.  Fig.  6 is  a polar 
view  of  the  egg  shown  in  Fig.  4. 

Fig.  7.  Section  through  an  egg  of  four  hours,  showing  the  peripheral  columns, 
distinctly  cut  off  in  most  regions  from  the  central  yolk. 

Fig.  8.  A portion  of  the  egg  in  Fig.  7,  more  enlarged. 

Fig.  9.  Projection  of  an  egg  with  eight  nuclei. 

Figs.  10-14.  Surface  views  of  successive  stages  of  surface  division. 

Fig.  15.  Section  of  an  egg  in  early  segmentation  showing  cleavage  planes  at  one 
pole  of  the  egg. 

Fig.  16.  Projection  of  an  egg  with  twenty-six  nuclei. 

Fig.  17.  Enlarged  view  of  a superficial  cell  in  early  segmentation  showing  the 
peripheral  protoplasm  (blastema)  and  protoplasmic  processes  extending  down  be- 
tween the  blastomeres. 

Fig.  18.  Egg  at  the  close  of  early  segmentation,  before  the  differentiation  of  ecto- 
mesoderm. 

Fig.  19.  Section  of  an  egg  during  the  process  of  delamination. 


66 


KINGSLEY. 


DESCRIPTION  OF  PLATE  V. 

Figs,  i,  2.  Sections  (longitudinal  and  transverse)  through  a portion  of  the  liver 
and  ovary  of  a Limulus  four  inches  in  length,  showing  the  formation  of  the  primordial 
ova  and  the  intrusion  of  older  ova  between  the  ovarian  epithelium  and  tunica 
propria. 

Fig.  3.  Section  of  an  egg  one  hour  after  impregnation,  showing  a possible  polar 
globule. 

Figs.  4,  5,  6.  Surface  views  of  eggs  four  hours  after  impregnation,  showing  the 
peculiar  segmentation  of  the  surface  previous  to  true  segmentation.  Fig.  6 is  a polar 
view  of  the  egg  shown  in  Fig.  4. 

Fig.  7.  Section  through  an  egg  of  four  hours,  showing  the  peripheral  columns, 
distinctly  cut  off  in  most  regions  from  the  central  yolk. 

Fig.  8.  A portion  of  the  egg  in  Fig.  7,  more  enlarged. 

Fig.  9.  Projection  of  an  egg  with  eight  nuclei. 

Figs.  10-14.  Surface  views  of  successive  stages  of  surface  division. 

Fig.  15.  Section  of  an  egg  in  early  segmentation  showing  cleavage  planes  at  one 
pole  of  the  egg. 

Fig.  16.  Projection  of  an  egg  with  twenty-six  nuclei. 

Fig.  17.  Enlarged  view  of  a superficial  cell  in  early  segmentation  showing  the 
peripheral  protoplasm  (blastema)  and  protoplasmic  processes  extending  down  be- 
tween the  blastomeres. 

Fig.  18.  Egg  at  the  close  of  early  segmentation,  before  the  differentiation  of  ecto- 
mesoderm. 

Fig.  19.  Section  of  an  egg  during  the  process  of  delamination. 


Journ.  Morph.  Vol  V/f 


i 


68 


KINGSLEY, 


DESCRIPTION  OF  PLATE  VI. 

Fig.  20.  A part  of  Fig.  19  more  enlarged,  showing  the  process  of  delamination. 

Figs.  21-26.  Successive  stages  of  the  germinal  area  previous  to  the  formation  of 
the  appendages.  See  the  text. 

Fig.  27.  Budding  of  the  legs.  j 

Fig.  28.  An  unusual  form  of  embryo,  appendage  I.  not  yet  formed. 

Fig.  29.  The  germ  viewed  as  a transparent  object.  Appendages  I. -VI.  present. 
The  nervous  system  is  covered  by  circularly  arranged  nuclei,  the  centres  of  rapid 
cell  proliferation.  Outside  the  germinal  area  are  seen  (^ss)  segmentally  arranged 
structures  of  possibly  glandular  or  sensory  functions. 

Figs.  30,  31.  Two  surface  views  illustrating  the  transfer  of  the  mouth  backwards, 
accompanied  by  the  formation  of  the  stomodaeum. 

Fig.  32.  Appearance  of  the  embryo  before  the  distinction  of  cephalothorax  and 
abdomen  is  prominent. 

Fig.  33.  Side  view  of  a late  embryo,  the  abdomen  differentiated. 

Figs.  34,  35.  Dorsal  and  ventral  views  of  the  last  larval  stage  before  the  appear- 
ance of  the  telson.  R.  Takano,  del. 

Fig.  36.  Longitudinal  section  of  a stage  about  like  Fig.  21,  showing  the  primitive 
cumulus  and  its  central  spot. 

Fig.  37.  Early  stage  of  mesoderm  formation. 

Fig.  38.  Late  stage  of  same,  showing  primitive  groove  and  lateral  connection  of 
mesoderm  and  ectoderm. 

Fig.  39.  More  enlarged  view  of  primitive  groove. 


68 


KINGSLEY, 


} 


DESCRIPTION  OF  PLATE  VI. 

Fig.  20.  A part  of  Fig.  19  more  enlarged,  showing  the  process  of  delamination. 

Figs.  21-26.  Successive  stages  of  the  germinal  area  previous  to  the  formation  of 
the  appendages.  See  the  text. 

Fig.  27.  Budding  of  the  legs. 

Fig.  28.  An  unusual  form  of  embryo,  appendage  I.  not  yet  formed. 

Fig.  29.  The  germ  viewed  as  a transparent  object.  Appendages  I. -VI.  present. 
The  nervous  system  is  covered  by  circularly  arranged  nuclei,  the  centres  of  rapid 
cell  proliferation.  Outside  the  germinal  area  are  seen  (j^)  segmentally  arranged 
structures  of  possibly  glandular  or  sensory  functions. 

Figs.  30,  31.  Two  surface  views  illustrating  the  transfer  of  the  mouth  backwards, 
accompanied  by  the  formation  of  the  stomodaeum. 

Fig.  32.  Appearance  of  the  embryo  before  the  distinction  of  cephalothorax  and 
abdomen  is  prominent. 

Fig.  33.  Side  view  of  a late  embryo,  the  abdomen  differentiated. 

Figs.  34,  35.  Dorsal  and  ventral  views  of  the  last  larval  stage  before  the  appear- 
ance of  the  telson.  R.  Takano,  del. 

Fig.  36.  Longitudinal  section  of  a stage  about  like  Fig.  21,  showing  the  primitive 
cumulus  and  its  central  spot. 

Fig.  37.  Early  stage  of  mesoderm  formation. 

Fig.  38.  Late  stage  of  same,  showing  primitive  groove  and  lateral  connection  of 
mesoderm  and  ectoderm. 

Fig.  39.  More  enlarged  view  of  primitive  groove. 


Journ  Morph.  Vol.VIf. 


THE 


I . 

Embryology  of  Limulus 


(PART  II.) 


BY 

J.  S.  KINGSLEY 


Reprinted  from  Journal  of  Morphology,  Vol.  VIII.,  No.  2 


BOSTON 

OINN  & COMPANY 

1893 


Volume  VIII. 


May,  i8gj. 


Number  2., 


JOURNAL 

OF 

MORPHOLOGY. 


THE  EMBRYOLOGY  OF  LIMULUS.  — PART  II. 

J.  S.  KINGSLEY. 


CONTENTS. 

Page 

Introduction 196 

The  Mesoderm  and  its  Derivatives 196 

CCELOM 197 

Comparisons 200 

Nephridia 203 

Comparisons 206 

Muscles.. 208 

Entosternite 209 

Organs  of  Circulation 209 

Comparisons 212 

The  Alimentary  Canal 214 

Mesenteron  214 

Stomod^eum 217 

Proctodeum 219 

Comparisons 219 

The  Nervous  System 222 

The  Respiratory  Organs 222 

Comparisons 225 

The  Relationships  of  Limulus 227 

The  Classification  of  the  Arthropoda 247 

Bibliography 255 

Explanation  of  Plates 260 


195 


196 


KINGSLEY, 


[VOL.  VIII. 


Introduction. 

The  preceding  portion  of  this  paper  (this  Journal,  Vol.  VII, 
p.  33)  dealt  with  the  habits,  ovigenesis,  the  origin  of  the  germ 
layers,  and  the  development  of  the  external  form  of  Limulus 
polyphemtLs,  together  with  some  more  general  questions  con- 
nected with  the  matters  under  discussion.  In  the  present  part 
the  development  of  some  of  the  organs  is  followed  out,  and  in 
conclusion  I have  considered  the  bearings  of  the  facts  upon  the 
systematic  affinities  of  the  Xiphosura  and  upon  the  classification 
of  the  Arthropods. 

I would,  before  beginning,  call  attention  to  the  fact  that, 
disregarding  the  neuromeres,  I have  numbered  the  somites  and 
the  appendages,  beginning  with  the  first  appendage.  This  may 
account  for  some  apparent  discrepancies  later,  and  it  also 
brings  my  article  into  harmony  with  those  of  other  writers. 
At  present  we  are  in  a transition  condition,  and  no  other 
course  seems  advisable.  The  matter  of  the  neuromeres  will 
be  discussed  in  its  proper  place. 

The  Mesoderm  and  its  Derivatives. 

As  previously  described  the  mesoderm  arises  by  cell  prolif- 
eration from  a median  longitudinal  line  on  what  will  eventually 
form  the  ventral  surface  of  the  embryo.  For  this  line  of 
mesodermal  formation  the  name  primitive  streak,  borrowed 
from  vertebrate  embryology,  seems  especially  appropriate. 
From  this  streak  {cf.  Figs.  42,  43)  the  middle  layer  grows 
outward  on  either  side  between  ectoderm  and  entoderm  (yolk), 
the  process  of  differentiation  beginning  in  front  and  gradually 
extending  backwards,  where  the  process  continues  for  some- 
time after  it  has  ceased  anteriorly  and  after  the  coelomic 
pouches  have  formed  in  the  first  somites  of  the  body.  This 
fact  gives  confirmation  of  the  correctness  of  interpretation  of 
anterior  and  posterior  in  the  earliest  embryos  (vide  ’92,  p.  47), 
a point  upon  which  absolute  certainty  was  impossible,  since 
the  features  there  described  only  become  visible  upon  the  em- 
ployment of  methods  which  coagulate  the  albumen  and  conse- 
quently kill  the  egg. 


No.  2.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


197 


At  first  the  mesoderm  forms  a continuous  sheet  across  the 
egg,  but  one  or  two  cells  in  thickness,  and  united  to  the  ecto- 
derm in  the  line  of  the  primitive  streak  (Figs.  42,  43);  and 
frequently  there  is  also  apparently  a marginal  connection  be- 
tween the  two  layers,  the  line  of  junction  being  indicated  by 
a groove^  (Figs.  42,  44).  This  line  of  union  is  apparently 
secondary,  and  I interpret  it  as  a precocious  differentiation  of 
dorsal  and  ventral  surfaces.  I have  seen  no  evidence  of  an 
augmentation  of  mesoderm  by  cell  proliferation  in  this  region. 

With  the  appearance  of  metamerism  the  connection  between 
mesoderm  and  ectoderm  is  lost,  except  in  the  stomodaeal  re- 
gion (Fig.  43)  and  in  the  abdominal  portion  of  the  embryo 
where  new  cells  continue  to  be  added  to  the  mesoderm  for 
some  little  time.  At  first,  after  the  separation  of  the  two 
layers  (Fig.  44),  the  mesoderm  extends  as  an  unbroken  sheet, 
one  or  two  cells  in  thickness,  across  the  germinal  area,  the- 
ectoderm  outside  this  area  losing  its  columnar  character  and 
becoming  more  flattened.  As  yet  there  is  no  appearance  of 
coelomic  cavities. 

So  far  as  my  observations  show  metamerism  obtains  its  first 
expression  in  the  ectoderm.  Thus  Fig.  40  represents  a longi- 
tudinal section  to  one  side  of  the  median  line  (where  the  coelom 
first  appears)  of  an  embryo,  which  in  surface  view  showed  dif- 
ferentiated cephalic  and  caudal  areas,  separated  by  a single 
somite.  The  boundaries  of  these  regions  are  recognizable  in 
the  ectoderm  of  the  section  marked  by  arrows,  while  the  meso- 
derm shows  no  corresponding  metamerism. 

CcELOM.  — Owing  to  difficulties  of  manipulation  I have  not 
been  able  to  correlate  surface  views  and  sections,  and  so  can- 
not say  exactly  how  many  somites  are  outlined  when  the 
coelom  first  appears.  It  is,  however,  preceded  by  a splitting- 
of  the  mesodermal  sheet  into  right  and  left  halves,^  a condition 
which  is  maintained  {cf.  Fig.  48)  until  after  the  appearance 
of  the  limbs.  By  this  splitting  are  produced  two  mesodermal 
bands,  the  inner  margins  of  which  are  undulating.  As  shown 

1 This  has  already  been  noticed  and  figured  by  Patten  (’90,  p.  37  5)  who  suggests 
a comparison  with  similar  appearainces  in  vertebrates. 

2 Except  in  oral  and  caudal  regions. 


198 


KINGSLEY. 


[VOL.  VIII. 


by  Fig.  45,  which  is  a sagittal  section  through  an  embryo  with 
seven  differentiated  somites,  the  mesoderm  extends  farther 
toward  the  middle  line  in  a segmental  than  in  an  interseg- 
mental  area.  Had  the  section  passed  further  from  the  middle 
line  the  mesoderm  would  have  formed  a continuous  sheet  from 
head  to  caudal  region. 

The  coelom  arises  (Fig.  43)  by  a splitting  of  the  lateral 
halves  of  the  mesoderm,  a pair  of  cavities  being  thus  formed 
for  each  somite.  These  coelomic  cavities  long  remain  distinct 
from  each  other,  and  the  dissepimental  walls  persist  until  a 
later  stage  of  development.  The  later  history  of  the  coelom  is, 
however,  very  difficult  to  follow  on  account  of  the  subsequent 
appearance  of  numerous  lacunae  in  the  mesoderm,  which,  so 
far  as  my  observations  go,  have  no  connection  with  the  prim- 
itive cavities  which  I would  homologize  with  the  coelom  of 
other  forms.  The  coelomic  pouches  appear  in  the  six  thoracic 
segments  before  they  do  in  those  of  the  abdomen  (Fig.  46). 
An  especially  interesting  fact  is  that  there  is  no  preoral 
coelomic  pouch,  but  the  cavities  of  the  first  postoral  somite 
send  prolongations  (Fig.  47)  into  the  region  in  front  of  the 
mouth. 

The  walls  of  the  coelom  differ  in  thickness.  The  somato- 
plure  is  usually  several  cells  thick,  while  the  splanchnoplure 
is  only  one  cell  in  thickness  and  rapidly  takes  the  form  of  a 
thin  layer  of  pavement  epithelium  closely  applied  to  the  under- 
lying yolk.  A partial  exception  to  this  general  statement 
occurs  in  the  abdominal  region  where  (Fig.  49)  the  somato- 
plure  at  first  may  also  be  a single  cell  thick.  With  the 
appearance  of  the  limbs,  as  I pointed  out  in  my  earlier  paper 
(’85,  p.  532),  the  coelom  extends  into  these  members,  but  it  is 
soon  excluded  by  the  rapid  growth  of  mesoderm  in  their 
interior.  Of  the  fates  of  the  coelomic  pouches  I cannot  speak 
with  absolute  certainty.  The  following  is  what  appears  to  me 
the  probable  history.  In  all  of  the  thoracic  segments  a portion 
(if  not  all)  of  the  coelom  is  gradually  carried,  with  the  advancing 
mesoderm,  from  the  ventral  on  to  the  dorsal  surface  of  the 
embryo.  That  a portion  is  thus  carried  is  certain  ; but  the 
rapid  formation  of  lacunar  spaces  in  the  somatoplure  renders 


No.  2.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


199 


it  impossible  to  say  whether  a portion  is  left  in  the  ventral 
region  of  each  segment.  That  a portion  does  so  persist  in  the 
fifth  segment  will  appear  later. 

Until  after  the  splitting  of  the  chorion  only  eight  pairs  of 
schizocoelia  are  produced,  there  being  a pair  for  each^  (Fig.  46) 
cephalothoracic  and  for  the  two  anterior  abdominal  somites. 
At  first  these  cavities  are  flat,  and  broader  than  long  ; the  first 
pair,  however,  rapidly  elongates  and  sends  a diverticulum 
forward  beneath  the  brain  on  either  side  of  the  oesophagus, 
into  the  preoral  region.  At  first  all  of  the  cavities  are  distinct, 
and  their  walls  epithelial  in  character,  but  soon  it  becomes 
difficult  to  follow  their  fate  with  certainty  since  a secondary 
splitting  of  the  mesoderm  soon  produces  a large  number  of 
anastomosing  lacunar  cavities  ie.g.  Figs.  53  bs.,  59  lac.,  etc.) 
connected  later  with  the  vascular  system  and  developing  into 
the  so-called  “body-cavity”  of  the  Arthropod,  the  existence  of 
which  much  confuses  the  sections. 

With  the  growth  at  first  laterally,  then  dorsally  and  medially, 
of  the  two  halves  of  the  mesoderm,  a portion  (if  not  all)  of 
the  coelom  in  somites  II,  III,  IV,  VI  and  VII,  and  a part  of 
that  in  somite  V,  is  carried  towards  the  dorsal  median  line  of 
the  embryo,  where,  in  the  latest  stages  I have  studied,  the 
cavities,  now  run  together,  persist  as  a longitudinal  tube 
(Figs.  63-67)  beneath  the  pericardial  sinus,  on  either  side  of 
the  heart  and  its  anterior  arterial  prolongation.  Posteriorly 
this  paired  cavity  does  not  at  any  time  extend  further  back 
than  somite  VIII  or  IX.  Whether  in  any  of  the  somites  all 
of  the  coelom  is  thus  carried  to  the  dorsal  surface  I am  unable 
to  say,  while  I have  not  followed  the  fate  of  the  coelom  of 
somite  I.  According  to  Kishinouye  the  anterior  coelomic 
pouches  are  pushed  inward  with  the  advancing  stomodaeum,  and 
hence  give  rise  to  the  splanchnoplure  of  the  oesophagus  and 
proventriculus.  In  somites  I-IV,  and  also  in  somite  VI,  I am^ 
unable  to  recognize  any  ventral  cavity  as  distinctively  coel9(m 
much  later  than  the  time  when  motion  is  seen  in  the  append- 
ages. In  the  posterior  abdominal  somites  the  coelom  persists 

iV 

1 According  to  Kishinouye  there  is  no  coelom  in  somites  II,  III,  ‘IV  of  the 
Japanese  Limulus. 


200 


KINGSLEY. 


[VOL.  VIII. 


on  the  ventral  side  until  a later  date,  and  is  then  apparently 
obliterated  by  a flattening  of  the  cavities  and  a growing  to- 
gether of  their  walls.  In  the  fifth  somite,  however,  I can 
speak  with  more  confidence, — for  here  the  coelomic  cavities 
divide  into  two  moieties,  a dorsal  and  a ventral,  and  the  latter 
remains  upon  the  ventral  surface  and  gives  rise,  in  a way  soon 
to  be  described,  to  the  nephridia. 

Concerning  the  fate  of  the  dorsal  portion  of  the  coelom  I 
am  uncertain.  It  persists,  as  has  been  said,  as  a perfectly 
distinct  cavity  with  epithelial  walls  on  either  side  of  the  central 
circulatory  organ  in  the  latest  stages  which  I have  studied. 
From  its  position  and  from  its  posterior  termination  I am 
inclined  to  think  that  this  portion  of  the  coelomic  epithelium 
is  finally  converted  into  the  reproductive  organs.  In  young 
Limuli  an  inch  and  a half  in  length  I have  found  no  traces  of 
the  cavity  except  as  it  might  be  represented  in  the  gonads. 
I regret  also  that  I have  not  been  successful  in  tracing  the 
history  of  the  reproductive  ducts.  There  are,  however,  so 
many  lacunae  developed  in  the  genital  somite  that  I have  not 
been  able  to  follow  the  fate  of  the  lower  portion  of  the  coelom 
in  that  somite. 

From  this  point  the  subsequent  history  of  the  mesoderm 
is  best  followed  under  the  headings  of  the  different  organs, 
but  before  taking  them  up  it  is  well  to  consider  the  facts 
already  described. 

Comparisons.  — The  only  previous  papers  ^ dealing  with  the 
mesoderm  of  Limulus  are  those  by  Patten  (’90),  Kishinouye 
(’91,  ’92)  and  myself  (’85),  and  Patten’s  remarks  are  only  inci- 
dental to  the  discussion  of  an  entirely  different  question. 
According  to  him  there  is  a short  slit-like  invagination  at 
the  posterior  end  of  the  embryo,  and  from  the  walls  of  this 
inpushing  much  of  the  entoderm  and  mesoderm  is  produced, 
Essentially  as  described  above  for  mesoderm  alone.  Again 

A Possibly  an  exception  should  be  made  made  in  favor  of  Dr.  Packard’s  last 
paper,  where  (’85,  p.  269)  a few  statements  are  made  concerning  this  layer.  He 
recognises  (Fig.  3)  two  coelomic  pouches  in  the  region  in  front  of  the  first  pair 
of  appen\Iages;  but  his  figures  clearly  show  that  he  has  not  seen  the  true  coelom; 
the  cavities  described  being  either  lacunar  or  artifacts.  The  coelom  does  not 
exist  in  the  plane  of  his  section  in  the  stage  he  has  studied. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


201 


Patten  describes  an  immigration  of  ectoderm  cells  from  differ- 
ent portions  of  the  germinal  area  ; and  lastly,  the  middle  layer 
receives  further  accessions  from  the  marginal  groove  already 
referred  to  above.  The  cells  of  this  marginal  area  are  de- 
scribed as  containing  an  extremely  long,  coiled,  brilliantly 
retractile  filament  ; and  some  of  these  cells  become  elongated 
to  form  the  dorsal  muscles,  the  filament  forming  the  longi- 
tudinal striation. 

I have  not  seen,  either  in  surface  views  or  in  sections,  the 
slit-like  invaginations  described  by  Professor  Patten  ; I have 
seen  no  additions  to  the  mesoderm  from  various  points  of  the 
blastoderm,  outside  of  the  limits  of  the  primitive  streak  ; I 
have  not  seen  any  cells,  much  less  great  masses  of  them, 
separate  from  the  primitive  streak  and  wander  into  and  become 
scattered  through  the  yolk  ; and  I have  yet  to  see  the  peculiar 
origin  of  the  dorsal  muscle-cells  which  he  describes.  He 
figures  (p.  374  Fig.  18  JO,  pstr.)  a cylindrical  rod  of  invaginative 
tissue,  an  appearance  lacking  in  my  sections,  and  refers  to 
masses  of  entoderm  cells  at  the  inner  end  of  the  oesophagus 
which  are  quickly  absorbed  and  which  I am  confident  do  not 
exist. 

Kishinouye  (’92)  has  the  mesoderm  in  Limithts  longispina 
arising  from  three  sources  : — from  the  cells  forming  the  lower 
part  of  the  blastoderm  thickening  ; from  the  primitive  streak  ; 
and  from  the  yolk  cells.  The  first  portion  forms  the  mesoderm 
of  the  cephalothorax  ; the  second  the  mesoderm  of  the  abdo- 
men ; the  last  probably  the  blood  corpuscles.  I must  be  per- 
mitted to  express  my  scepticism  upon  this  account  of  the 
origin  and  fate  of  the  different  portions  of  the  middle  layer. 
In  the  subsequent  history  however  we  are  in  close  accord  and 
only  the  points  of  difference  need  be  noted.  The  order  of  the 
appearance  of  the  mesoderm  somites  is  the  same  and  in  them 
the  coelom  appears  as  schizocoelia.  According  to  Kishinouye 
no  coelomic  cavities  appear  in  somites  II,  III,  or  IV,  and  in 
the  others  they  do  not  extend  into  the  appendages.  In  both 
points  the  Atlantic  Limulus  differs  ; cavities  occurring  in 
every  postoral  somite  (Fig.  46)  and  at  first  extending  into  the 
limbs.  In  both  species  the  coelom  (or  at  least  a part  of  each 


202 


KINGSLEY. 


[VOL.  VIII. 


cavity)  migrates  to  the  dorsal  surface  where  it  comes  to  lie  at 
either  side  of  the  central  circulatory  organ.  Kishinouye  says 
it  disappears  before  hatching.  I do  not  find  that  it  does  so 
but  find  it  persisting  {cf.  Fig.  87)  after  the  assumption  of  the 
adult  characters,  for  a portion  of  the  length  of  the  body.  My 
belief  that  it  gives  rise  to  the  gonads  was  referred  to  above. 

Our  knowledge  of  mesoderm  formation  in  the  Arachnida  is 
deficient,  but  as  far  as  it  goes  it  agrees  well  with  that  of  Limulus. 
Thus  all  accounts  agree  in  the  following  : The  mesoderm  arises 
by  a proliferation  from  a median  ventral  primitive  streak,  the 
proliferation  continuing  later  in  the  posterior  region  than  in 
front  ; next  the  mesoderm  (-f  entoderm  in  Euscorpius)  separates 
from  the  ectoderm  and  forms  at  first  a broad  sheet  across  the 
ventral  surface  of  the  embryo  ; and,  next,  this  sheet  becomes 
divided  into  right  and  left  bands  connected  behind  ; and  into 
distinct  somites  in  either  half  of  the  body.  Later  in  each  half 
of  each  somite  a coelomic  cavity  is  formed  by  splitting  and  these 
cavities  extend  temporarily  into  the  corresponding  cephalotho- 
racic appendages.  This  will  apply  equally  well  to  Limulus, 
but  farther,  differences  in  details  are  to  be  noted.  Thus  Balfour 
(’80,  p.  174,  PI.  XX,  Fig.  13)  describes  cells  from  the  yolk 
migrating  into  and  adding  to  the  mesoderm,  while  in  Euscorpius 
(Laurie  ’90)  the  mes-entoderm  is  first  differentiated  from  the 
ectoderm  and  then  later  it  is  divided  into  mesoderm  and  ento- 
derm. From  what  we  now  know  of  Arachnid  development  I 
think  it  safe  to  regard  the  view  of  Balfour  that  the  mesoderm 
was  increased  by  migrations  from  the  yolk  as  erroneous, 
certainly  his  figure  (’80,  PI.  XX,  Fig.  13)  does  not  require  such 
interpretation,  while  the  differences  in  scorpions  is  readily  un- 
derstood in  connection  with  the  peculiar  features  of  the  for- 
mation of  the  entoderm  already  discussed  (Part  I,  p.  53  ff.) 

Both  Laurie  (Scorpion)  and  Balfour  (Agalaena)  describe  a 
prestomial  coelom  but  without  stating  whether  it  actually  be- 
longs to  the  prestomial  area  or  migrates  to  that  region  as  in 
Limulus.  Kowalevsky  and  Schulgin,  on  the  other  hand  (’86) 
describe  in  the  scorpion  a preoral  somite  with  a distinct  cavity. 
Laurie  has  also  studied  the  genital  duct  which  according  to  him 
develops  from  the  coelom  of  somite  VII.  Owing,  however, 


THE  EMBRYOLOGY  OF  LI M ULUS. 


No.  2.] 


203 


to  its  late  appearance  he  is  not  certain  as  to  its  nephridial 
nature. 

Nephridia  (so-called  Coxal  Glands).  — As  has  been  described 
above,  a portion  of  the  coelom  of  the  fifth  somite  remains  upon 
the  ventral  side  of  that  somite,  while  the  other  coelomic  cavities 
are  migrating  toward  the  dorsal  surface  (Fig.  52).  For  a con- 
siderable time  this  cavity  shows  no  changes  worthy  of  remark, 
and  first  in  about  Stage  G,  (Fig.  32)  is  any  modification  notice- 
able. The  cavity  now  begins  to  elongate  and  to  become  bent 
upon  itself  like  the  letter  U,  the  rounded  portion  being  directed 
anteriorly.  The  U now  grows  in  length  and,  the  posterior 
end  being  fixed,  soon  extends  into  somite  IV.  With  this 
change  in  shape  a differentiation  in  the  epithelial  cells  be- 
comes noticeable,  most  of  them  becoming  cubical  or  columnar, 
while  those  upon  the  inner  side  of  the  inner  end  of  the  U 
retain  their  pavement  character. 

These  changes,  just  before  Stage  H,  produce  the  following 
results  (Figs.  56-59):  The  coelom  (of  somite  V)  is  now  di- 
vided into  two  portions,  (i)  a true  coelomic  portion  (the  “end 
sac”  of  authors.  Fig.  58  nstl)  bounded  by  pavement  epithelium 
on  its  inner  side,  and  passing  directly  into  the  nephrostomial 
portion  with  columnar  cells  ; and  (2)  the  nephridial  duct  which 
passes  forward  (Fig.  57),  as  the  proximal  limb  of  the  organ,  to 
the  loop  (Fig.  56)  and  then  backward,  as  the  other  limb  (Figs. 
57j  59  nd.),  to  the  posterior  limits  of  somite  V.  As  yet 

it  has  no  connection  with  the  exterior,  but  in  the  last  section 
(Fig.  59)  can  be  seen  an  inpushing  of  ectoderm  {no.),  which  is 
to  form  its  external  aperture.  In  the  various  sections  in  the 
neighborhood  of  the  coxal  gland  at  this  stage  may  be  seen 
numerous  lacunae  {lac.)  in  the  mesoderm,  which  is  rapidly  as- 
suming the  trabecular  condition  characteristic  of  the  later 
stages.  I have  never  been  able  to  certainly  trace  any  con- 
nection between  these  lacunae  and  the  coelomic  cavity  and  am 
strongly  of  the  opinion  that  none  exists. 

In  Stage  H (Fig.  54  a-i ; Fig.  55,  reconstruction)  the  same 
conditions  are  seen,  except  that  the  duct  now  opens  to  the 
exterior.  We  have  here  the  end  sac  {c  5)  which  passes  directly 
into  the  nephrostome  without  any  sharp  line  of  demarcation 


204 


KINGSLEY. 


[VOL.  VIII. 


(except  that  of  change  in  the  character  of  the  cells)  and  this 
in  turn  into  the  two  Ijmbs  of  the  duct.  In  this  series  of  sec- 
tions a peculiarity  is  seen,  which  in  most  cases  does  not  make 
an  appearance  until  a much  later  stage,  viz.  the  formation  of 
trabeculae  of  mesodermal  tissue  which  invade  the  cavity,  and 
passing  from  wall  to  wall  of  the  proximal  portion  of  the  organ 
tend  to  subdivide  it  and  give  it  an  anastomosing  character. 

The  changes  which  have  occurred  up  to  the  Stage  I are 
represented  in  the  horizontal  sections  (Figs.  6i  a-e)  and  the 
sagittal  section  (Fig.  62).  Figs.  61  f and  g are  reconstruc- 
tions by  projection  of  the  whole  series  of  which  a few  sections 
are  represented  in  61  a-e.  The  tube  has  now  become  more 
elongate,  extending  in  front  nearly  to  the  anterior  limits  of 
somite  IV,  while  the  two  limbs  are  relatively  much  more 
closely  applied  to  each  other  than  before.  With  this  growth 
the  regions  are  much  more  differentiated.  The  end  sac  (Fig. 
6\  d)  is  separated  from  the,  now  numerous,  lacunae  of  the 
mesoderm  by  a layer  of  pavement  epithelium  which  in  my 
series  nowhere  shows  a break.^  This  coelomic  sac  passes 
directly,  as  before,  into  the  nephrostome  and  this  again 

into  the  duct,  which  shows  no  change,  except  increase  in 
length,  until  in  the  distal  limb,  when  about  at  the  level  of  the 
end  sac  it  becomes  enlarged  into  an  excretory  vesicle  or 
bladder,  ev.  (Figs.  60  and  61  f,  g). 

Later  than  Stage  I,  I have  comparatively  few  notes  which 
add  to  the  information  contained  in  my  earlier  paper  (’85)  and  in 
the  simultaneous  one  by  Mr.  Gulland  (’85).  One  reconstruction, 
however,  demands  attention  ; that  represented  in  Fig.  62. 
This  is  the  nephridium  in  Stage  K just  before  the  molt  in 
which  the  telson  appears  in  the  adult  form.  One  error  how- 
ever is  noticeable  in  it  ; the  lateral  amplification  is  too  great 
and  consequently  the  diameter  of  the  tubes  and  the  extent  of 
the  outgrowths  are  exaggerated.  In  this  the  same  parts  are 
recognizable  as  before  but  they  have  undergone  some  modifica- 
tions. Thus  in  the  region  of  the  end  sac  a fenestration  is 

1 In  my  earlier  paper  I was  in  doubt  upon  this  point  and  thought  that  possibly 
(’85,  p.  535)  I had  found  such  an  opening,  which  led  to  the  reconstruction  of  the 
tube  with  an  internal  funnel.  Still  I was  not  positive.  I now  feel  confident  that 
the  opening  of  that  paper  was  an  artifact. 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  205 

observable,  the  beginning  of  the  anastomosing  condition  of  the 
adult  {cf.  Fig.  55);  while  the  proximal  (internal)  limb  is 
thrown  into  a series  of  four,  outwardly  directed  diverticula 
which  are  segmentally  arranged  and  which  occupy  somites  two 
to  five.  The  external  (distal)  limb  shows  fewer  modifications, 
the  chief  being  that  the  excretory  vescicle  is  relatively  smaller 
than  before  ; the  external  opening  still  persists  and  I have  not 
found  out  when  it  closes.  This  figure  corresponds  quite  closely 
to  Gulland’s  (’85)  Fig.  3,  except  that  in  his  reconstruction  the 
outgrowths  from  the  proximal  limb  are  represented  as  taking  a 
sagittal  direction  and  the  anastomosing  character  of  the  inner 
end  is  carried  still  farther,  a condition  doubtless  due  to  the 
older  stage  with  which  he  worked. 

Although  I have  not  followed  it  out  the  appearance  at  this 
stage  gives  countenance  to  the  view  that  the  whole  organ  of 
the  adult  is  derived  from  the  coelom  of  somite  V and  that  the 
apparently  metameric  lobes  figured  by  Packard  (’80,  PI.  Ill, 
Fig.  7,  copied  by  Lankester  ’84  p.  153  Fig.  3)  are  not  the 
remnants  of  the  nephridia  of  the  corresponding  somites  but 
are  rather  the  derivatives  of  the  diverticula  shown  in  my  re- 
construction. Besides  an  increase  in  the  size  of  the  lobes  all 
that  is  necessary  to  convert  my  reconstruction  into  the  “coxal 
gland”  of  the  adult  are  closure  of  the  external  opening,  more 
or  less  complete  fusion  of  the  two  limbs  of  the  duct,  accom- 
panied by  an  increase  in  the  anastomoses,  the  result  being  to 
convert  coelom  and  duct  into  the  spongy  tissue  of  the  adult, 
the  whole  organ  being  a series  of  anastomosing  tubes,  the 
“caeca”  of  Lankester.  This  view  of  the  morphology  of  the 
organ  receives  confirmation  from  the  fact  that  I have  seen  no 
fusion  of  coelomic  spaces  two  to  five  while  Kishinouye,  as 
already  stated,  finds  no  coelom  at  all,  in  the  anterior  somites 
which  are  later  invaded  by  the  coxal  gland. 

A word  as  to  the  external  opening  of  the  nephridium.  Gulland 
(’85,  p.  513)  states  that  it  is  “at  the  base  of  the  coxa  of  the  fifth 
limb  on  the  side  next  the  fourth  appendage  and  on  the  dorsal 
surface.”  I find  upon  repeated  examination  that  it  is  rather 
upon  the  posterior  side  of  the  coxa  of  the  fifth  pair  of  legs,  as 
I stated  it  in  my  earlier  paper  (’85). 


206 


KINGSLEY. 


[VOL.  VIII. 


Comparisons.  — The  foregoing  account  differs  in  some  points 
from  that  which  I gave  in  1885.  I then  failed  to  recognize  the 
genetic  connection  between  the  nephridium  and  the  coelom  and 
also  failed  to  recognize  the  closed  condition  of  these  organs. 
The  history  of  our  knowledge  of  the  nephridia  in  Limulus  can 
be  briefly  summarized. 

These  organs  were  first  noticed  by  Packard  (’75^)  who  from 
their  histology  and  by  exclusion,  concluded  that  they  were 
renal  in  function  and  homologous  with  the  green  gland  of 
Crustacea.  Five  years  later  (’80)  he  redescribed  and  figured 
their  adult  structure  and  suggested  a comparison  in  their 
position  with  the  shell  gland  of  the  Entomostraca.  Lankester 
in  1882  described  these  organs  and  compared  them  with  the 
coxal  glands  of  the  scorpion  and  later  (’84)  gave  the  histology 
with  some  detail.  In  both  papers  he  was  inclined  to  compare 
them  with  the  green  gland  of  the  Crustacea  but  still  admitted 
the  possibility  or  even  probability  of  their  being  ectodermal 
or  entodermal  with  a frame  work  of  ‘‘ skeletotrophic  tissue.” 
In  the  next  year  Gulland  (’85)  described  the  organ  in  young 
specimens  while  Kingsley  at  the  same  time  gave  an  account  of 
the  early  history  of  the  organ,  comparing  it  exactly  with  the 
shell  gland  of  the  Entomostraca  and  claimed  that  it  and  the 
genital  ducts  of  both  Crustacea  and  Arachnida  should  be 
regarded  as  Nephridia.  In  1890  Kingsley  stated  that  the 
coxal  gland  of  Limulus  was  derived  from  the  coelom  of  somite 
V,  that  it  terminated  csecally  and  gave  in  outline  the  above 
account  of  the  origin  of  the  segmentally  arranged  lobes.  Kish- 
inouye  (’91®)  stated  that  in  Limulus  longispinus  the  coxal  gland 
is  formed  from  the  ventral  part  of  the  coelom  of  somite  V 
and  later  (’91^)  describes  the  method  more  at  length.  His 
account  agrees  well  with  the  foregoing  excepting  that  the  out- 
growths of  the  metameric  lobes  occur  at  an  earlier  stage  and 
the  end  sac  (his  “funnel”)  is  in  the  mesoblastic  dissepiment 
between  the  fifth  and  sixth  appendage  bearing  segments. 

All  others  who  have  had  occasion  to  refer  to  the  nephridia 
of  Limulus  have  used  the  material  included  in  the  papers 
enumerated  above.  This  is  true  of  Eisig  (’88)  Loman  (’88)  and 
Sturanay  (’9i). 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


207 


Of  the  coxal  glands  of  the  Arachnids  we  know  considerable 
concerning  the  adult  structure  and  little  about  the  development. 
They  have  been  found  in  Scorpions,  Phalangids,  Solpugids, 
Acarina  and  Araneina.  They  occur  in  either  somite  III  or 
V or  they  may  co-exist  in  the  same  individual  in  two  somites 
at  the  same  time.  Usually  the  external  duct  becomes  closed, 
as  in  Limulus,  at  an  early  date  but  in  Phalangids  (Loman  ’88)  it 
remains  open  through  life. 

Laurie  (’90)  describes  the  development  of  the  coxal  gland 
in  Euscorpius  italicus.  In  the  youngest  stage  studied  it  is  a 
simple  straight  tube  opening  distally  to  the  exterior  and  prox- 
imally  by  a funnel  to  the  coelom  of  somite  V,  which  is  a much 
larger  space  than  in  Limulus.^  In  the  next  stage  the  duct 
becomes  bent  on  itself  so  that  it  appears  in  sections  cut  in 
three  places.  Its  connection  with  the  coelom  is  still  evident. 
Later  it  becomes  more  complicated  and  the  whole  gland 
becomes  enveloped  in  a thin  capsule  of  mesoderm  cells,  but  the 
process  is  not  further  described.  The  external  opening  persists 
until  after  hatching. 

Kishinouye  (’90)  has  studied  the  development  of  the  gland 
in  Agalaena,  Lycosa,  and  other  spiders.  In  these  it  occurs  in 
somite  III  and  is  described  as  consisting  of  a duct  of  ecto- 
dermal origin  which  breaks  through  to  the  coelom.  The 
schematic  figures  do  not  prove  this  origin  of  the  duct. 

Lebedinsky’s  account  (’92)  of  the  development  in  Phalangids 
is  most  complete.  The  first  appearance  of  the  nephridium  is  a 
weak  outgrowth  of  the  wall  of  the  coelom  of  somite  III. 
The  cells  of  this  outgrowth  become  columnar  while  its  external 
end  grows  into  connection  with  the  ectoderm  of  the  first 
ambulatory  appendage.  This  thickened  portion,  which  is  to 
form  the  duct  of  the  organ,  now  grows  inward,  carrying  the 
wall  of  the  coelom  with  it,  so  that  its  internal  end  is  surrounded 
by  a ridge.  This  inner  ingrowth  forms  the  nephrostome.  The 
ectoderm  is  resorbed  later,  giving  an  opening  to  the  exterior, 
and  the  tube  becomes  convoluted. 

1 Kowalevsky  and  Schulgin  (’86)  saw  the  organ  when  it  was  but  little  compli- 
cated ; they  describe  its  duct  as  ectodermal. 


2o8 


KINGSLEY. 


[VOL.  VIII. 


That  these  organs  in  Limulus  and  Arachnids  are  homologous 
admits  of  little  doubt.  It  is  scarcely  more  doubtful  that  they 
belong  to  the  same  category  as  the  antennal  glands  and  shell 
glands  of  the  Crustacea.  The  correspondence,  as  I earlier 
pointed  out,  is  exact  between  the  coxal  gland  of  Limulus  and 
the  shell  gland  of  the  Entomostraca.  Their  closure  in  Limulus 
and  certain  arachnids  is  paralleled  for  instance  in  the  case  of 
Argulus  (teste  Leydig,  B9),  where  the  shell  gland  is  functional 
only  in  early  life.  On'  the  other  hand,  as  stated  above,  the 
coxal  gland  in  Phalangids  is  functional  in  the  adult. 

In  the  light  of  recent  investigations  these  organs  must  be 
regarded  as  nephridia,  and  the  arguments  to  the  contrary 
advanced  by  Eisig  (’88)  are  without  foundation.  Two  recent 
studies  are  of  interest  here  : That  of  Sedgwick  on  the  develop- 
ment of  the  nephridia  in  Peripatus,  and  those  of  Weldon*  (’89 
and  ’9i)  on  the  relations  of  the  antennal  gland  to  the  coelom  in 
the  Decapods.  In  Peripatus  the  development  is  strikingly  like 
that  in  Limulus,  except  there  are  numerous  pairs  of  nephridia 
in  the  former.  In  both  there  is  the  formation  of  small  coelo- 
matic  spaces  ; in  both  {cf.  Kishinouye,  ’91^)  a division  of  the 
coelom  into  dorsal  and  ventral  portions,  and  in  both  the«con- 
version  of  the  ventral  coelom  into  end  sac,  nephrostome,  duct, 
and  bladder.  It  is  interesting  in  this  connection  to  note  that 
according  to  Loman  (’87)  in  Phalangium,  where  the  nephridial 
opening  does  not  close  and  the  organ  remains  functional 
through  life  the  Malpighian  tubes  — the  other  urinary  organs 
— are  lacking. 

I am  inclined  to  believe  that  the  genital  ducts  of  Limulus 
are  also  to  be  regarded  as  nephridia,  but  I have  searched  in 
vain  for  any  trace  of  their  development.  Laurie’s  account  of 
the  origin  of  the  ducts  in  the  scorpion  is,  as  he  says,  intelligible 
upon  the  ground  that  they  are  nephridia;  their  somewhat  tardy 
appearance  and  lateness  in  opening  to  the  exterior  not  seriously 
militating  against  such  a view. 

Muscles.  — I have  not  attempted  to  trace  the  history  of  the 
muscles  except  to  a slight  extent.  The  muscles,  which  move 
the  feet  and  which  extend  from  the  dorsal  surface  of  the  body 
down  to  the  appendages,  are  developed  along  the  interseg- 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  20g 

mental  lines.  The  tissue  from  which  they  arise  is  the  bound- 
aries of  the  somites  which  extend  inward  toward  the  median 
line  and  which  by  their  encroachment  into  the  yolk  outline 
the  liver  lobes.  The  early  history  of  this  portion  is  traced 
in  connection  with  the  alimentary  canal.  In  the  abdominal 
region  the  differentiation  of  the  muscles  of  the  gill  appendages 
is  accomplished  in  the  same  way,  and  it  is  interesting  to  note 
(see  Fig.  79)  that  the  anterior  wall  of  the  somite  develops 
the  extensor  and  the  posterior  the  flexor  of  the  corresponding 
appendage. 

Entosternite.  — Passing  between  the  alimentary  canal  and 
the  nervous  system  in  the  cephalothoracic  region,  and  serving 
at  the  same  time  to  connect  the  pedal  muscles  of  the  right 
and  left  sides  is  a layer  of  tissue  which  serves  as  a tendon, 
or  rather  as  a series  *of  tendons,  and  which  by  its  later  chondri- 
fication*  or  chitinization  (vide  Lankester,  ’84,  p.  133)  gives  rise 
to  the  entosternite.  It  is  to  be  noticed  that  in  its  develop- 
ment the  entosternite  (Figs.  74,  84,  86,  87  es)  is  always  fibrous 
and  it  arises  from  the  fibrous  tissue  of  the  region.  The  other 
“cartilages”  occurring  as  axial  tendons  in  the  gills  and  oper- 
culum (Fig.  79)  present  in  their  early  stages  a distinctly  chon- 
droid  appearance. 

Organs  of  Circulation.  — The  early  history  of  the  central 
circulatory  organ  — the  heart  — of  Limulus  was  outlined  in 
my  paper  of  1885.  Later,  Kishinouye  has  added  to  the  ac- 
count given  there  and  has  corrected  some  points  in  my  de- 
scription. So  this  early  history  need  not  be  detailed  here. 
The  heart  arises  as  a result  of  the  extension  of  the  mesoderm 
over  the  yolk  towards  the  dorsal  median  line.  Its  walls  are 
formed  by  the  edges  of  the  advancing  tissue,  and,  according 
to  Kishinouye,  as  the  walls  of  the  tube  thus  formed  are  inter- 
rupted in  the  intersomitic  region,  a series  of  segmentally 
arranged  openings  into  the  cavity  — the  ostia  — are  produced. 
The  differentiation  of  the  heart  begins  at  first  behind  and 
gradually  extends  forward.  In  its  early  formation  the  heart 
of  Limulus  affords  support  to  the  theory  of  Biitschli  (’82)  in 
regard  to  the  relations  of  the  circulatory  system  to  the  seg- 
mentation cavity. 


210 


KINGSLEY. 


[VOL.  VIII. 


My  present  description  begins  with  Stage  H (Fig.  64).  In 
this  the  heart  may  be  seen  with  walls  in  which  no  definite 
arrangement  of  cells  is  visible  and  with  two  blood  corpuscles 
in  its  interior.  It  is  connected  in  this  section  with  the  dorsal 
ectoderm  by  a cord  of  cells,  while  on  either  side  are  two 
cavities,  a dorsal  and  a ventral.  The  ventral  is  clearly  the 
coelom  of  the  somite,  and  its  walls,  somatoplure  and  splanch- 
noplure,  are  perfectly  distinct.  The  upper  cavity,  the  peri- 
cardium, is  plainly  lacunar,  and  is  produced  by  a splitting  of 
the  mesoderm,  and  at  this  early  stage  is  limited  distally  by 
the  trabecular  tissue  so  characteristic  of  the  embryo  of 
Limulus.  Farther  forward  (Fig.  66)  the  heart  is  larger,  and 
the  cells  of  its  walls  are  arranged  in  a single  layer. 

In  the  next  stage  (I,  Fig.  66)  the  somatopluric  mesoderm 
has  given  rise  to  the  alary  muscles  which  are  best  developed  in 
the  anterior  portion.  In  the  abdominal  region  (Fig.  67),  the 
heart  is  larger,  but  in  all  parts  it  as  yet  consists  of  the  single 
layer  of  cells  which  were  found  in  the  preceding  stage.  Fig. 
67,  which  passes  through  the  plane  of  the  genital  operculum, 
shows  on  either  side  the  posterior  extension  of  the  dorsal 
coelom.  A few  sections  further  back  (Fig.  68)  this  cavity  dis- 
appears from  the  sections.  In  longitudinal  section  (Fig.  82) 
the  heart  is  seen  to  extend  back  to  about  the  middle  of  the 
abdomen  and  forward  to  the  anterior  end  of  the  yolk,  following 
this  down  toward  its  junction  with  the  stomodaeum.  At  its 
anterior  end  the  heart  divides  into  two  aortic  arches  (the 
“crosses  aortiques  ” of  Alph.  Milne  Edwards,  ’73)  which  I 
have  called  the  sternal  arteries.  These  arteries  (Fig.  76)  pass 
down  one  on  either  side  of  the  stomodaeum  to  dispose  them- 
selves at  first  as  two  tubes  upon  the  upper  surface  of  the 
ventral  nerve  chain.  I have  not  satisfied  myself  of  the  way  in 
which  these  sternal  arteries  arise  but  the  observations  which  I 
have  made  are  not  incompatible  with  the  view  that  dorsally  at 
least  they  are  interseptal.  This  point  is  however  difficult  to 
settle  on  account  of  the  numerous  lacunae,  which,  as  already 
mentioned,  early  appear  in  the  mesoderm  and  confuse  the 
observer.  At  this  stage  (I)  no  other  arteries  arise  from  the 
heart. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


21  I 


In  Stage  K the  conditions  are  much  the  same,  the  relations 
of  the  anterior  end  of  the  heart  and  the  sternal  arteries  being 
shown  in  Fig.  77,  drawn  from  a wax  reconstruction. 

In  Stage  L,  the  heart  (Figs.  72,  73,  82)  has  nearly  attained 
its  adult  condition  so  far  as  segmentation  into  chambers  is 
concerned.  In  Fig.  73  — representing  a horizontal  section  — 
the  anterior  end  of  the  heart  is  shown,  enclosed  in  the 
pericardial  space  and  supported  by  the  alary  muscles.  In 
front,  on  either  side  are  the  roots  of  the  sternal  arteries  but  I 
have  not  seen  at  this  or  any  earlier  stage  the  frontal  arteries  of 
Milne  Edwards.  Fig.  72,  taken  at  a lower  level,  shows  the 
section  of  the  sternal  arteries  on  either  side  of  the  narrow  duct 
connecting  the  proventriculus  with  the  mesenteron. 

It  is  not  until  late  Stage  H that  the  sternal  arteries  reach 
the  nervous  system.  At  first  they  extend  themselves  as  two 
separate  tubes  along  the  dorsal  surface  of  the  nervous  cords 
and  extend  backwards  but  a slight  distance  upon  them  forming 
the  rudiments  of  the  neural  artery.  There  is  at  this  time  no 
trace  of  any  tube  beneath  the  nerves.  It  is  especially  inter- 
esting that  this  condition  which  is  transitory  in  Limulus  should 
be  permanent  in  the  Scorpion. 

In  Stage  I the  neural  artery  extends  back ’behind  the  middle 
of  the  cephalothorax  but  its  termination  is  indistinct.  In 
somite  VII  (Fig.  67)  no  traces  of  it  are  to  be  found*.  The 
partition  between  the  two  arteries  still  persists  (Figs.  70-71) 
but  on  either  side  the  artery  is  extending  itself  around  the 
nervous  system  and  appearing  beneath  it,  thus  giving  rise  to 
the  peculiar  condition  so  well  known  as  characteristic  of  the 
adult  horseshoe  crab.  This  condition  is  brought  about,  at 
least  in  part,  by  outgrowths  from  the  dorsal  tubes  but  whether 
there  be  cavities  formed  independently  beneath  the  cord  which 
are  later  taken  into  the  neural  artery  I cannot  say.  Most  of 
it  is  accomplished  by  the  downward  growth  and  the  wrapping 
of  these  portions  around  the  cord,  there  being  as  Milne  Edwards 
has  suggested  a soldering  of  the  two  edges  of  the  vessel  and  a 
subsequent  resorption  of  the  resulting  lamella.  As  will  be 
seen,  by  this  process  of  formation  the  nerve  does  not  float  freely 
in  the  blood  but  is  surrounded  by  a neurilemma  which  is  part 


212 


KINGSLEY. 


[VOL.  VIII. 


of  the  arterial  wall.  In  other  words  the  artery  is  not  morpho- 
logically inside  the  artery. 

In  Stage  K,  as  shown  by  the  reconstruction  Fig.  77,  this 
process  has  been  completed  at  the  anterior  end  of  the  ventral 
cord.  Below,  this  figure  shows  the  neural  artery  {av^j  repre- 
sented as  filled  with  a solid  mass,  while  the  omission  of  the 
nerve  cord  leaves  a central  cavity  from  which  proceed  the 
openings  for  the  nerves  on  the  sides.  In  front  (to  the  right) 
below,  the  forwardly  directed  process  shows  the  nature  of  the 
outgrowths  by  which  the  ventral  artery  is  extending  itself 
beneath  the  brain. 

In  Stage  L the  dorsal  portion  of  the  neural  artery  has  reached 
the  abdomen,  while  in  the  somite  of  the  fifth  appendage  the 
conditions  are  advanced  as  far  as  shown  in  Fig.  74.  In  the 
oldest  embryos  I have  sectioned  the  arteries  surrounding  the 
nerves  have  extended  themselves  into  the  appendages  (Fig.  89). 

I have  not  attempted  to  follow  the  development  of  the  blood 
sinuses,  etc.  They  occur  most  abundantly  in  the  abdominal 
region  (Figs.  65,  68,  73,  etc)  and  that  they  are  produced  by  a 
splitting  of  the  mesoderm  is  easily  seen  in  the  development  of 
the  gills.  The  pericardial  sinus  belongs  to  the  category  of 
these  blood  spaces,  the  coelom  taking  no  part  in  its  formation. 

Comparisons.  — I have  above  referred  to  Kishinouye’s  account 
of  the  formation  of  the  heart  in  Limulus  with  which  my  recent 
observations  are  in  fair  accord.  In  my  first  account  I described 
the  heart  as  arising  from  a solid  cord  of  cells  but  this  was  a 
mistake.  A re-examination  of  the  slide  showed  that  at  the 
stage  described  the  heart  was  already  formed  and  its  cavity  was 
filled  with  blood  corpuscles. 

In  the  Arachnida  most  observers  have  described  the  heart 
as  arising  from  the  coalescing  edges  of  the  somites,  meeting 
in  the  dorsal  median  line,  there  being  slight  differences  in  de- 
tails between  the  Scorpions  (Kowalevsky  and  Schulgin,  Laurie) 
and  the  Araneina  (Schimkewitsch,  Locy,  Morin,  Kishinouye). 
In  the  other  groups,  as  far  as  I am  aware,  no  detailed  obser- 
vations have  been  made. 

The  pericardium  of  the  spiders,  according  to  Schimkewitsch 
(’87)  arises  as  a layer  of  mesoderm  split  off  from  the  splanch- 


No.  2.] 


THE  EMBRYOLOGY  OF  L/MULUS. 


213 


noplure,  while  the  somatoplure  gives  rise  to  the  alary  muscles. 
If  this  be  so  it  is  an  important  point  of  difference  between 
Limulus  and  the  Arachnids.  Schimkewitsch  further  describes 
the  pulmonary  veins  arising  as  outgrowths  from  the  pericardium, 
the  lateral  arteries  as  outgrowths  (Ausstiilpulgen)  from  the 
heart  itself. 

Alimentary  Tract. 

The  alimentary  tract  of  Limulus,  like  that  of  all  Arthropods, 
consists  of  three  divisions  ; stomodseum  and  proctodseum,  of 
ectodermal  origin,  and  mesenteron  (including  the  “intestine” 
and  “liver”),  derived  from  the  entoderm.  These  parts  are 
easiest  considered  in  connection  with  each  other. 

Mesenteron.  — The  separation  of  the  entoderm  from  the 
ecto-mesoderm  by  delamination  was  described  (p.  46)  in  the 
first  part  of  the  present  article.  As  will  be  remembered,  I 
regard  the  whole  of  the  nucleated  yolk  after  that  separation 
as  the  true  entoderm.  From  the  time  of  differentiation  of 
this  layer  until  the  first  molt  there  is  but  slight  histological 
change  in  the  region  of  the  midgut  and  its  diverticula  aside 
from  a slight  increase  in  number  and  consequent  decrease  in 
size  of  the  yolk  (=  entoderm)  cells.  There  is,  however,  a 
very  considerable  change  in  the  shape  of  the  entoderm  which 
may  be  summarized  as  follows  : — 

When  the  entoderm  is  first  separated  from  the  rest  of  the 
egg,  it  is,  like  the  egg,  spherical  in  outline.  It  then  becomes 
gradually  flattened  {cf.  Fig.  82)  and  more  and  more  ovoid  in 
outline,  viewed  from  above,  corresponding  in  this  with  the 
changes  in  shape  of  the  embryo.  As  a result  there  may 
soon  be  distinguished  a large  semicircular  mass  of  entoderm 
in  the  cephalothorax  and  a smaller,  more  cylindrical  portion 
in  the  abdomen.  Coincidently  with  this  change  in  outline 
the  beginning  of  the  differentiation  of  midgut  and  “liver” 
occurs.  As  the  mesoderm  extends  itself  peripherally  from 
the  median  ventral  line  of  the  embryo,  it  gives  rise  to  slight 
intersegmental  ridges,  the  septa  of  Balfour.  Until  this  cen- 
trifugal growth  reaches  its  extreme  these  septa  are  slight  in 
extent,  but  as  it  attains  the  margin  of  the  carapax  and,  turning 


214 


KINGSLEY. 


[VOL.  VIII. 


on  to  the  dorsal  surface,  begins  to  grow  back  to  the  dorsal 
median  line,  there  begins  a rapid  centripetal  growth  of  these 
septa,  resulting  in  broad  sheets  of  tissue  which  divide  the 
peripheral  portion  of  the  yolk  into  a corresponding  number 
of  lobes,  those  of  the  first  division  being  of  course  segmentally 
arranged.  Thus  there  are  in  the  cephalothoracic  region  six 
pairs  of  these  lobes,  while  in  the  abdomen  they  are  less  dis- 
tinct and  less  extensive  and  are  only  temporary,  disappearing 
totally  at  an  early  stage.  The  fact  of  their  temporary  ap- 
pearance in  this  region  is,  however,  of  considerable  interest. 

A similar  process  of  lobulation  of  the  yolk  has  been  de- 
scribed by  several  authors  for  various  Arachnids,  and  it  may 
be  regarded  as  characteristic  of  the  large-yolked  eggs  of  the 
group.  It  however  occurs  to  a greater  or  less  extent  in  other 
forms.  Thus,  in  the  Crustacea  the  lobulation  of  the  midgut 
gland  (“liver”)  is  of  the  same  character,  while  in  the  leeches, 
as  described  by  Dr.  Whitman  (’78),  the  differentiation  of  the 
intestine  and  its  diverticula  is  exactly  the  same.  Were  this 
process  of  differentiation  of  liver,  lobes  and  intestine  to  go 
on  regularly,  it  would  result  in  the  production  of  a Limulus 
with  a paired  liver  in  each  somite,  each  half  emptying  by  its 
own  duct  directly  into  the  intestine.  This,  however,  does 
not  occur.  With  the  development  of  the  extensive  muscular 
system  of  the  gill-bearing  appendages  and  the  large  blood 
sinuses  in  that  region  the  abdominal  midgut  diverticula  disap- 
pear. In  the  cephalothorax  the  primitive  regularity  shows 
the  following  modifications.  The  septa  do  not  all  grow  at 
the  same  rate,  and  (Fig.  83)  some  are  interrupted  at  points 
in  their  growth,  so  that  two  or  more  lobes  remain  in  direct 
connection  with  each  other.  This  occurs  between  lobes  i,  2 
and  3,  and  also  between  lobes  4,  5 and  6.  At  the  same  time 
the  inner  ends  of  the  septa  become  expanded  by  the  develop- 
ment of  the  muscles  of  the  feet  so  that  they  in  places  run 
together,  cutting  off  lobes  i and  2,  and  5 and  6 from  direct 
connection  with  the  central  mass.  In  this  way  the  six  primary 
liver  lobes  and  the  two  hepatic  ducts  {hep.,^  hep. '2')  of  either 
side  of  the  adult  are  differentiated.  A later  peripheral  in- 
growth of  mesoderm  still  farther  divides  up  the  primary 


THE  EMBRYOLOGY  OF  LIMULUS. 


No.  2.] 


215 


lobes  into  lobules  (Figs.  34,  35)  resulting  in  the  adult  condi- 
tion. 

At  the  anterior  end  of  the  body  an  ingrowth^  similar  to 
the  septa  carries  back  the  anterior  end  of  the  intestine,  and 
intervenes  to  separate  the  first  pair  of  lobes  from  each  other. 
With  this  ingrowth  this  pair  of  lobes,  which  at  first  were  at 
right  angles,  come  to  lie  parallel  to  the  principal  axis  of  the  body. 

The  central  unsegmented  part  of  the  yolk  which  remains 
after  the  differentiation  of  the  ‘Tiver”  forms  the  “intestine” 
of  the  adult.  It  extends  from  the  point  of  the  first  appearance 
of  the  stomodseum  back  to  the  posterior  end  of  the  body. 

Until  after  the  first  molt  after  hatching  the  entoderm  retains 
the  same  histological  characters  which  it  had  at  its  first  differ- 
entiation. It  is  a mass  of  yolk  without  lumen  and  is  divided 
into  a number  of  polygonal  cells  with  clearly  marked  cell  walls 
and  central  nuclei.^  Excepting  in  a slight  difference  in  size, 
it  is  impossible  to  distinguish  histologically  between  the  ento- 
derm cells  of  Stages  C and  I.  That  some  change  does  occur 
in  the  interval,  of  a chemical  rather  than  of  a histological 
character,  is  shown  by  the  fact  that  while  in  the  earlier  stages 
the  yolk  is  very  difficult  to  section,  in  the  later  it  cuts  as 
readily  as  any  other  tissue  of  the  body. 

After  the  molt  which  produces  the  adult  form  (Stage  L)  the 
histiogenesis  of  the  epithelium  of  the  midgut  and  its  diverticula 
begins.  It  appears  first  in  the  intestine  and  later  in  the  liver  ; 
and  in  the  intestine  it  is  first  seen  at  the  anterior  end  (Fig.  81). 
From  the  study  of  numerous  sections  {cf.  Figs.  81,  85,  88)  the 
process  is  clearly  seen  to  be  a direct  conversion  of  the  yolk- 
cells  into  the  epithelial  lining  of  the  mesenteron.  In  Fig.  85 
— a sagittal  section  through  the  junction  of  stomodaeum  and 
mesenteron  at  early  stage  L — the  entoderm  cells,  en,  near  the 

1 For  clearness  this  and  the  lateral  septa  are  considered  as  ingrowths,  but  they 
are  to  a large  extent  outgrowths  as  well,  since  the  margin  of  the  carapax  is 
farther  removed  from  the  median  line  in  the  later  than  in  the  earlier  stages,  and 
it  is  coincidently  with  this  change  in  the  relative  position  of  the  margin  of  the 
body  that  the  septa  are  developed.  This  is  even  more  marked  in  front  than  at 
the  sides  of  the  body. 

2 In  my  figures  the  yolk  is  represented  as  solid,  neither  cell  walls  or  nuclei 
being  shown.  They  are,  however,  very  distinct  in  all  of  my  preparations. 


KINGSLEY. 


[VOL.  VIII. 


216 

middle  line,  are  seen  to  have  assumed  a columnar  character 
and  to  be  nearly  free  from  yolk,  while  on  either  side  they  pass 
into  a tissue  crowded  with  yolk  spherules  {ys)  in  which  the  cell 
boundaries  cannot  be  followed  and  in  which  the  nuclei  are 
irregularly  arranged.  In  Fig.  84  — a transverse  section  of  a 
slightly  older  embryo  — the  same  conditions  are  shown  upon  a 
smaller  scale.  On  the  upper  left  side  of  the  intestine  {mes.)  the 
cells  have  a well-marked  epithelial  character,  while  on  either 
hand  they  pass  directly  into  the  normal  yolk  cells  of  the  earlier 
condition.  In  a slightly  older  individual  (Fig.  81)  the  whole 
anterior  end  of  the  intestine  is  free  from  yolk,  and  its  lining 
cells  (represented  diagrammatically)  have  the  character  of  a col- 
umnar epithelium,  while  at  the  posterior  end  they  pass  directly 
into  the  yolk  cells  which  still  fill  the  whole  cavity  in  this  region. 

This  rearrangement  of  the  epithelium  is  well  advanced  in 
the  intestine  before  it  begins  in  the  liver,  and  it  advances  more 
rapidly  in  the  central  than  in  the  peripheral  parts  of  the  latter. 
It  thus  forms  first  the  epithelium  of  the  hepatic  ducts  (Figs. 
72,  86)  and  then  the  secretory  epithelium.  In  the  latter  I have 
failed  to  recognize  an  early  differentiation  of  purely  epithelial 
and  excretory  cells  such  as  has  been  described  in  some  spiders. 

As  will  be  seen,  I regard  the  yolk  in  Limulus  from  the  time 
of  its  delamination  as  true  entoderm.  I fail  to  recognize,  at 
least  here,  the  existence  of  “ vitellophags  ” whose  purpose  is 
merely  the  metabolization  of  the  yolk  and  which  then  degen- 
erate. I look  upon  the  yolk  cells  from  the  beginning  as 
morphologically  a true  epithelium,  the  cells  of  which,  being 
gorged  with  yolk,  are  crowded  from  their  proper  position,  thus 
obliterating  the  lumen  and  obscuring  their  true  nature.  In 
the  later  stages,  when  there  is  a rapid  development  of  tissues, 
there  is  a corresponding  call  upon  the  entodermal  structures  for 
nourishment.  Then  it  is  that  the  yolk  cells  act  temporarily  as 
“ vitellophags  ” and,  metabolizing  the  yolk,  pass  the  products  on 
to  the  other  tissues.  It  is  only  then  that,  the  yolk  being  out  of  the 
way,  they  are  able  to  rearrange  themselves  as  a true  epithelium. 

While  this  view  is  in  full  accord  with  the  observations  of 
most  students  of  Arachnid  development,  it  is  at  variance  with 
some  of  the  commonly  received  ideas  of  Arthropod  embryology. 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  21  J 

which  are  to  the  effect  that  the  yolk  cells  are  “ vitellophags,” 
degenerating  and  not  contributing  to  the  formation  of  the 
entodermal  epithelium  which  arises  from  cells  derived  from 
some  other  source.  I feel  confident  that  this  is  not  the  case 
in  Limulus.  I have  yet  to  see  any  evidence  of  degeneration 
of  the  yolk  cells,  and  further,  I have  seen  no  cells  other  than 
those  of  the  yolk  which  could  supply  the  epithelium  of  intestine 
and  liver.  From  the  very  time  when  the  yolk  is  first  included 
in  a mesodermal  envelope  this  layer  is  to  be  clearly  traced  (sp.) 
as  a splanchnoplure  closely  enveloping  the  yolk,  and  in  the 
later  stages  the  same  layer  is  seen  (Figs.  84,  85)  in  exactly  the 
same  place  and  presenting  the  same  conditions.  In  some 
sections  which  I have  made  of  Hexapod  eggs  I have  seen 
appearances  which  lead  me  to  think  that  possibly  in  these 
forms  the  entoderm  of  authors  is  in  reality  splanchnoplure. 
The  fact  that  most  observers  have  closed  their  investigations 
before  the  development  of  a well-differentiated  entodermal 
epithelium  leaves  a gap  which  renders  their  interpretations  not 
conclusive  on  this  point. 

Stomod^um. — As  already  described  (Vol.  VII.  p.  52)  the 
anterior  end  of  the  primitive  streak  is  marked  by  a spot  where 
the  cells  are  deeper  and  more  columnar  (Figs.  43,  45,  mo.) 
than  elsewhere  in  the  median  line,  and  this  spot  is  usually,  in 
Arthropods,  regarded  as  marking  the  position  of  the  future 
mouth,  and  all  discussions  of  the  transfer  of  the  mouth  from 
a preappendicular  position  to  one  behind  the  first  pair  of  ap- 
pendages are  concerned  with  the  connection  of  this  spot  with 
the  mouth  of  the  adult.  In  reality  this  spot  marks  the  junc- 
tion of  stomodseum  and  mesenteron  and  forms  the  inner  end 
of  the  foregut.  Hence  to  call  it  the  mouth  is  to  introduce 
an  element  of  confusion. 

At  the  time  of  the  first  outlining  of  the  limbs  the  invagina- 
tion of  the  stomodaeum  begins,  and  the  process  is  one  which 
finds  its  closest  parallel  in  the  invagination  of  the  neural  canal 
in  the  vertebrates.^  At  first  it  is  a shallow  pit  with  small 

1 It  is  needless  to  say  that  this  affords  no  foundation  for  the  curious  vagaries 
of  Gaskell,  as  to  the  origin  of  the  vertebrate  nervous  system  from  the  alimentary 
tract  of  the  Arthropod. 


2i8 


KINGSLEY. 


[VOL.  VIII. 


lumen  and  with  an  external  opening  pear-shaped  in  outline, 
narrower  in  front  and  wider  behind.  It  is  in  fact  enclosed  by 
two  ridges,  one  on  either  side,  while  posteriorly  it  is  without 
distinct  boundaries  and  passes  directly  into  the  ventral  ecto- 
derm. The  lateral  walls  gradually  unite  in  front  {cf.  Vol.  VII, 
Figs.  30,  31),  and  are  at  the  same  time  added  to  behind.  It 
is  by  the  continuation  of  this  process  that  the  stomod^um  is 
invaginated  and  the  mouth  comes  to  occupy  a position  behind 
the  first  pair  of  appendages.  Thus  it  will  readily  be  seen  that 
with  regard  to  Limulus,  at  least,  Claus  is  wrong  when  he  says 
(’87,  p.  129)  that  the  preoral  condition  of  the  appendages  is 
“nicht  ein  Lagenwechsel  des  Mundes  . . . sondern  eine  im 
Laufe  der  Entwicklung  vollzogene  Aufwartsbewegung  der 
Gliedmassen  mit  entsprechenden  Verschiebung  der  Ursprungs- 
stelle  des  zugehorigen  Nerven.”  It  would  seem  probable  that 
the  same  conditions  obtain  in  the  Crustacea,  but  detailed  ob- 
servations are  as  yet  lacking. 

At  first  the  cells  of  the  stomodaeum  invaginated  in  this  man- 
ner form  a low  cubical  epithelium,  but  they  soon  elongate  and 
assume  the  columnar  character  which  is  found  in  this  region 
in  all  the  later  stages.  At  an  early  date  they  also  begin  the 
secretion  of  the  chitinous  cuticle.  At  the  outset  all  of  the 
stomodaeum  is  apparently  formed  by  this  invagination,  and  the 
tube  is  straight  between  mouth  and  the  inner  end.  Later  it 
begins  to  elongate  by  interstitial  growth,  and  in  this  way  a 
flexure  in  the  sagittal  plane  is  produced  {cf.  Figs.  81,  82). 
This  flexure  is  also  increased  by  the  flattering  of  the  em- 
bryo. 

With  the  introduction  of  the  flexure  there  begins  a differ- 
entiation of  the  stomodaeum,  at  first  into  an  internal  proven- 
triculus  (stomach  of  most  authors)  and  an  outer  tube.  Later, 
the  latter  in  turn  is  subdivided  into  buccal  cavity  and  oesoph- 
agus proper  (Fig.  81)  In  the  proventriculus  there  soon 
develop  longitudinal  folds,  and  before  the  connection  of  stomo- 
daeum with  mesenteron  is  effected,  the  proventriculus  has, 
except  in  size,  the  characters  which  it  has  in  the  adult. 

After  the  epithelium  of  the  midgut  has  been  formed  at 
the  anterior  end,  the  connection  between  fore-  and  midgut  is 


No.  2.] 


THE  EMBRYOLOGY  OE  LI M ULUS. 


219 


effected  by  a breaking  down  of  the  wall  between  them.  At 
first  the  proventriculus  empties  directly  into  the  mesenteron, 
but  later  (Fig.  72)  the  inner  end  of  the  former  becomes  drawn 
out  in  a slender  tube  which  projects  slightly  as  the  ‘‘cone” 
into  the  intestines.  The  limits  of  the  two  regions,  ectodermal 
and  entodermal,  at  this  stage  are  clearly  distinguished  by  the 
chitinous  cuticle  upon  the  former. 

Proctodeum. — In  striking  contrast  to  its  development  in  the 
Crustacea  the  proctodaeum  in  Limulus  is  late  in  its  appearance 
and  small  in  extent.  As.  late  as  Stage  I (Fig.  82)  it  appears 
merely  as  a slight  inpushing  of  the  ectoderm  upon  the  ventral 
surface.  In  Stage  L it  is  wider  but  scarcely  deeper  than  be- 
fore (Fig.  81).  In  still  older  specimens  (Fig.  88)  the  boundary 
between  mesenteron  and  proctodaeum  has  broken  through  and 
the  now  more  elongate  proctodaeum  has  become  thrown  into 
inner  folds.  At  the  point  of  juncture  between  ectoderm  and 
entoderm  there  appears,  above  and  below  in  the  section,  an 
enlargement  of  the  lumen  of  the  tube  and  a second  similar 
enlargement  occurs  within  the  entodermal  portion  of  this  tract. 
As  to  the  meanings  of  these  enlargements  I have  nothing 
to  offer  aside  from  the  fact  that  in  connection  with  them 
the  Malpighian  tubules  of  the  Hexapods  and  the  analogous 
structures  of  the  Arachnids  naturally  suggest  themselves. 

Comparisons.  — Almost  nothing  was  previously  known  of  the 
development  of  the  alimentary  tract  of  Limulus.  In  1885  I 
gave  essentially  the  same  account  of  the  formation  of  the  stomo- 
daeum  and  the  differentiation  of  the  liver-lobes,  illustrating 
both  with  diagrammatic  figures.  In  the  same  year  Brooks  and 
Bruce  in  their  preliminary  paper  (’85)  describe  the  entodermal 
epithelium  as  arising  from  the  yolk  cells  in  the  same  way  as  I 
have  done  and  they  further  say,  though  without  any  details, 
that  the  stomodaeum  arises  as  an  ingrowth  which  at  first  goes 
upward  and  forward  and  then  bends  upon  itself.  Though  not 
specifically  the  same  in  words  the  account  given  by  Kishinouye 
(9i)  of  the  development  of  the  stomodaeum  is  easily  brought 
into  harmony  with  the  foregoing,  and  especially  interesting  is 
his  statement  (pp.  79-80)  that  “As  the  upper  lip  grows  pos- 
teriorly, the  mouth  opening  which  was  at  first  pre-appendicular 


220 


KINGSLEY. 


[VOL.  VIII. 


gradually  shifts  its  position  backward.”  Kishinouye  has  nothing 
to  offer  concerning  mesenteron  or  proctodaeum. 

In  the  Arachnida  almost  all  observers  describe  conditions 
closely  similar  to  those  obtaining  in  Limulus.  Since  the  time 
of  Balfour’s  paper  (’80)  the  liver-lobes  have  been  recognized  as 
differentiated  by  the  ingrowth  of  mesodermal  septa  into  the 
yolk  in  the  same  manner  as  in  Limulus,  and  the  hepatic  ducts 
(of  course  different  in  number)  as  formed  in  substantially 
the  same  way.  Balfour,  while  thinking  that  certain  obser- 
vations possibly  pointed  to  the  origin  of  the  hepatic  epithelium 
from  the  cells  of  the  thickened  ends  of  the  septa,  still  recog- 
nized the  entoderm  in  the  yolk.  Balfour  did  not  trace  the 
formation  of  the  entodermal  epithelium  but  later  authors  agree 
that  it  arises  in  the  spiders  from  the  yolk  cells,  the  differenti- 
ation beginning  at  first  at  the  posterior  end.  So  far  as  their 
observations  go  Locy,  Morin  (the  text  of  his  preliminary  paper, 
the  copies  of  the  figures  of  his  later  article  in  Russian),  and 
Schimkewitsch  (’8?)  agree  in  the  recognition  of  the  yolk  nuclei 
of  spiders  as  the  nuclei  of  the  future  entoderm.  Later,  Schim- 
kewitsch (’9o)  changes  his  views  ; he  now  recognizes  as  ento- 
derm in  the  “ Tracheates,”  the  smaller  cells  which  lie  on  the 
ventral  surface  of  the  yolk,  while  the  mass  of  the  yolk  cells  in 
the  Arachnids  are  the  Anlage  of  the  blood  corpuscles.  Such  a 
view  is  impossible  with  Limulus,  since  before  the  yolk  cells 
undergo  any  change  they  are  entirely  enclosed  in  the  splanchno- 
pleural  layer  of  the  mesoderm,  through  which  migration  to 
the  blood  vessels  is  impossible. 

Faussek,  in  his  account  of  the  development  of  the  Phalangids 
(’9i)  says  that  at  the  close  of  the  embryonic  period  the  yolk 
cells  (derived  as  in  Limulus  by  delamination)  divide  rapidly, 
and,  with  a small  amount  of  protoplasm,  begin  to  throw  them- 
selves down  upon  the  mesodermal  envelope  of  the  midgut  and 
its  diverticula.  At  first  these  patches  of  entodermal  epithelium 
are  irregularly  scattered  but  they  soon  begin  at  the  anterior 
end,  to  arrange  themselves  in  the  cylindrical  epithelium  of  this 
region. 

In  the  scorpions  the  conditions  are  quite  different.  As 
already  pointed  out  (Vol.  VII,  pp.  55  ff.)  Kowalevsky  and 


No.  2.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


221 


Schulgin  (’86)  and  Laurie  (’90)  show  that  in  the  early  stages  the 
entoderm  is  a distinct  differentiation  from  the  germinal  area ; 
it  remains  for  a long  time  as  a solid  mass  at  the  posterior  end 
of  the  embryo  and  only  later  spreads  itself  out  as  a definite 
layer  enclosing  the  yolk  and  forming  the  epithelium  of  the 
midgut  and  its  diverticula.  Of  the  details  of  the  formation  of 
the  liver-lobes  but  little  is  said,  but  apparently  it  occurs  through 
the  ingrowth  of  the  mesodermal  septa  as  in  Araneina  and  in 
Limulus.  The  different  type  of  formation  of  the  entoderm  in 
the  scorpions  and  Limulus  is  possibly  the  strongest  objection 
which  can  be  raised  to  the  close  association  of  the  two  forms, 
for  while  the  two  types  can  be  reconciled,  it  implies  an  ex- 
tremely long  separation  of  the  forms.  Still  it  must  be  borne 
in  mind  that  if  this  be  advanced  as  an  objection  it  must  be 
equally  valid  in  proving  that  the  Araneina  and  the  scorpions 
are  but  remotely  connected,  a thesis  which  I hardly  care  to 
defend,  while  on  the  other  hand  it  would  show  closer  relation- 
ship between  Limulus  and  the  spiders  than  between  the  former 
and  the  scorpions,  a view  which  is  negatived  by  numerous 
other  facts  of  structure  and  ontogeny. 

Limulus  also  agrees  with  the  Arachnida  in  the  early  appear- 
ance and  elongation  of  the  stomodseum  and  the  shortness  and 
lateness  of  appearance  of  the  proctodaeum.  As  yet,  no  obser- 
vations are  recorded  as  to  the  manner  of  invagination  of  the 
stomodaeum,  but  as  figured  by  Morin  (in  Korschelt  and  H eider 
’92)  and  Locy  (’86)  it  bears  close  resemblance  to  that  of  Limulus 
in  its  bending  and  in  the  early  differentiation  of  a terminal 
pouch  clearly  comparable  to  the  proventriculus  of  Limulus.  In 
the  only  sagittal  section  given  by  Laurie  of  the  later  stage  of 
the  stomodaeum  in  the  scorpion  (’90,  PI.  XVII,  Fig.  48)  the 
proventricular  enlargement  is  not  very  apparent  but  the  buccal 
cavity  is  well  marked.  Concerning  the  proctodeal  region  the 
comparisons  reveal  little  of  importance.  In  the  spiders  we 
have  in  this  region  the  formation  of  the  stercoral  pocket  ^ 
which  is  without  parallel  in  Limulus  {vide,  however,  supra  p. 
219  and  Fig.  88). 

1 Kishinouye’s  observation  (’90,  p.  68)  that  the  stercoral  pocket  is  developed 
from  the  unpaired  coelom  of  the  caudal  region  certainly  needs  confirmation. 


222 


KINGSLEY. 


[VOL.  VIII. 


Nervous  System. 

I leave  all  questions  concerning  the  nervous  system  and 
sense  organs  untouched,  except  in  so  far  as  is  necessary  to 
explain  some  points  which  will  appear  later.  I do  this  the 
more  willingly  since  my  friend,  Dr.  Wm.  Patten,  has  for  several 
years  been  devoting  especial  attention  to  this  system.  I find  in 
Limulus  that  at  an  early  stage  the  nervous  system,  viewed  from 
the  surface,  presents  the  appearance  of  numerous  circular  pits 
(Fig.  29).  These,  which  are  shown  in  section  in  Figs.  49  and 
65,  I suppose  to  be  what  Patten  (’89  p.  602)  refers  to  in  his 
statement  that  “the  central  cord  and  brain  of  Arthropods,  is 
at  first  composed  entirely  of  minute  sense  organs,  which  in  the 
scorpion  have  the  same  structure  as  the  segmental  ones  at  the 
base  of  the  legs.”  Kishinouye  has  also  seen  the  same  struct- 
ures and  compares  them  to  the  ommatidia  of  the  eye.  There  is, 
however,  no  such  differentiation  of  the  nuclei  as  are  shown  in 
the  figures  of  the  latter.  Unlike  Patten,  I interpret  these  in- 
pushings of  the  nuclei  as  centres  for  the  rapid  proliferation  of 
nerve  cells,  and  not  as  sense  organs.  I also  am  inclined  to 
withdraw  my  original  account  of  the  formation  of  segmental 
sense  organs  (’90),  as  I am  now  inclined  to  believe  that  the 
structures  which  I described  as  sensory  in  structure  (various 
figures  “i-i-”)  are  more  probably  glandular.  The  brain  arises 
from  three  pairs  of  ganglia  in  front  of  the  pair  which  inner- 
vates the  first  pair  of  appendages.  I believe  these  three  ganglia 
to  represent : the  first,  the  primitively  preoral  nerve  centre,  the 
homologue  of  the  “brain”  of  the  annelids;  the  other  two  to 
belong,  like  the  deuto-  and  tritocerebrum  of  the  Hexapod,  to 
ganglia  which  have  left  the  postoral  and  have  wandered  into 
the  preoral  region.  Upon  the  real  state  of  affairs  I have,  how- 
ever, almost  no  actual  observations,  and  base  my  opinions  largely 
upon  the  conditions  in  other  groups. 

Respiratory  Organs. 

As  is  well  known  the  respiratory  organs  of  Limulus  are 
borne  upon  five  pairs  of  appendages  situated  on  somites  VIII- 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  223 

XII.  Each  of  these  appendages  has  upon  its  posterior  surface 
a large  number  of  rounded  quadrilateral  lamellae  (the  number 
varying  with  age),  the  ^‘gill-leaves,”  and  the  whole  organ  is 
known  as  the  “gill-book.”  The  first  of  these  appendages  to 
appear  is  the  most  anterior  one  (VIII).  It  grows  out  from  the 
body,  not  as  a cylindrical  process  like  appendages  I-VI,  but  as 
a broad  lobe  with  an  oblique  insertion  upon  the  ventral  surface 
of  the  body  (Figs.  28,  32  g).  At  first  this  lobe  consists  merely 
of  a fold  of  ectoderm  (Fig.  78)  marked  off  from  the  ventral 
surface  by  an  impushing  behind,  and  containing  in  its  interior 
scattered  branching  mesoderm  cells  between  which  are  numer- 
ous large  lacunae.  At  stage  H this  appendage  shows  the  first 
appearance  of  the  gill  lamellae  in  the  form  of  a single  fold  of 
the  ectoderm  of  the  posterior  surface  at  about  midway  of  the 
length  of  the  member.  Very  soon  a second  and  smaller  out- 
growth appears  between  the  first  leaf  and  the  body,  and  so  on 
in  regular  succession  (Fig.  79)  the  new  leaves  continuing  to  be 
added  at  the  base  of  the  appendage,  the  outer  leaves  being  the 
larger  and  older. 

When  the  first  gill-leaf  appears  on  appendage  VIII,  appendage 
IX  is  budded,  and  on  this  the  first  gill-leaf  appears  (Fig.  79) 
when  on  appendage  VIII  there  are  four  or  five  lamellae.  The 
subsequent  growth  of  these  appendages  and  the  new  gill-leaves 
is  but  a repetition  of  that  already  given.  At  the  molt  with 
which  the  telson  appears  and  with  which  my  studies  end, 
appendage  X has  appeared,  but  is  as  yet  without  lamellae,  while 
appendages  VII  and  IX  are  well  provided  in  this  respect  (Fig. 
80). 

At  stage  I (Fig.  79)  the  gills  and  gill  appendages  present 
the  following  appearances  in  their  finer  structure.  The  ec- 
toderm has  become  columnar  and  has  secreted  on  its  free  surface 
a thin  cuticula.  In  the  gill-leaves  the  two  walls  are  united 
here  and  there  by  fine  mesodermal  filaments  which  extend 
between  the  two  walls  like  the  tie  rods  of  an  architectural 
structure.  Between  these  trabeculae  are  lacunae  without  definite 
walls,  and  in  these,  scattered  blood  corpuscles  are  to  be  seen, 
showing  that  these  spaces  are  in  connection  with  the  general 
circulatory  system.  These  lamellae  are  shown  in  cross  section 


224 


KINGSLEY. 


[VOL.  VIII. 


in  Fig.  68,  where  also  may  be  seen,  at  either  side  of  the  gill- 
leaf  larger  lacunae,  forming  the  afferent  and  efferent  blood 
channels  of  the  lamella. 

In  the  gill  appendage  as  in  the  operculum  (appendage  VII) 
is  a central  rod  of  compact  tissue  somewhat  resembling  car- 
tilage, but  whose  fate  I have  not  traced.  In  my  former 
paper  (’85,  PI.  XXXIX,  Fig.  38)  the  line  from  the  abbreviation 
for  muscle,  was  by  mistake  of  the  lithographer,  run  to  this 
structure.  On  either  side  of  this  rod  are  developed  the  muscles 
of  the  appendages.  They  have  their  origin  in  a narrow  line  on 
either  side  of  the  back  (Fig.  67)  and  are  inserted  in  the  wall  of 
the  appendage  at  the  lower  end  of  the  rod  just  referred  to. 
In  transverse  section  these  muscles  (Fig.  67)  are  seen  to  be 
fan-shaped,  the  line  of  insertion  embracing  nearly  the  entire 
width  of  the  appendage.  From  its  origin  and  insertion  the 
muscle  in  front  of  the  rod  is  seen  to  be  antagonistic  to  that 
behind  and  we  may  consider  the  two  as  respectively  flexor  and 
extensor  in  function. ^ The  flexors  of  one  somite  and  the 
extensors  of  the  next  have  their  origins  closely  approximate, 
and  their  traction  soon  results  in  the  drawing  inwards  a small 
patch  of  the  dorsal  ectoderm,  thus  producing  in  the  adult  the 
line  of  depressions  on  either  side  of  the  middle  of  the  tack, 
and  the  corresponding  chitinous  ingrowths  in  the  interior  of 
the  same  region.  One  of  these  ingrowths  of  chitin  secreting 
ectoderm  is  cut  across  in  Fig.  68  ent. 

In  the  last  stage  studied  (Fig.  80)  the  conditions  are  essen- 
tially the  same  as  before,  except  that  the  gill-leaves  are  larger 
and  more  numerous.  The  section,  however,  does  not  pass  in 
the  right  plane  to  show  the  muscles  and  internal  rod.  For  a 
description  of  this  section  I cannot  refrain  from  quoting  Mac- 
Leod’s (’87,  p.  4)  description  of  the  lung-book  of  a scorpion: — 
“Nous  trouvons  en  [la  figure]  la  coupe  d’un  certain  nombre  de 
fines  lamelles,  les  lamelles  pulmonaires,  placees  horizontalement, 
fibres  a leur  extremite  posterieure  ou  caudale,  c’est-a-dire  la 
plus  rapprochee  de  1’ extremite  caudal  de  I’animal,  vers  la  droite 

1 I have  been  unable  to  identify  with  certainty  these  muscles  with  those  of  the 
adult  as  described  by  Benham  (’83).  At  this  early  age  they  are  not  differentiated 
as  they  are  later. 


No.  2.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


225 


de  la  figure,  et  attachees  en  avant.  Entre  ces  lamelles  se 
trouvent  des  cavites  en  forme  de  fentes  qui  communiquent  en 
arriere  avec  une  cavite  generale  laquelle  debouche  a son  tour 
a I’exterieure  par  une  fente  stigmatique.” 

Comparisons.  — The  only  previous  accounts  of  the  gill  de- 
velopment are  by  myself  (’85)  and  Kishinouye  (’91,  p.  72),  and 
the  foregoing  differs  from  them  in  being  somewhat  fuller  in 
some  details  (Kishinouye  calls  the  metastoma  an  appendage^ 
hence  his  appendage  IX  is  my  VIII,  etc.).  The  process  is  ex- 
tremely simple  — the  outgrowth  of  lamellate  processes  from 
the  posterior  surfaces  of  the  corresponding  appendages,  the 
newer  lamellae  being  formed  proximally  — and  yet  my  early 
account  does  not  seem  to  have  satisfied  Laurie,  who  says  (’9i, 
p.  137)  “a  detailed  account  of  the  development  of  these  ap- 
pendages Limulus  may  throw  more  light  on  the  matter”  of 
the  homologies  of  the  respiratory  organs  of  the  scorpion  and 
the  king  crab.  Had  not  Laurie  been  confused  by  some  strange  ^ 
ideas  of  the  homologies  I think  that  my  previous  account 
would  have  proved  detailed  enough  for  his  purposes,  for  the 
two  organs,  — the  gill  of  Limulus  and  the  lung  of  Scorpio, — 
can  be  compared  in  detail  in  the  simplest  manner,  without  the 
invocation  of  any  inversion,  of  any  “ parabranchial  stigmata,” 
of  any  conversion  of  air  space  into  blood  space,  or  the  like. 

Several  workers  have  described  the  development  of  the  res- 
piratory organs  of  the  Arachnida,  and  from  the  papers  of 
Metschnikoff  (’71),  Kowalevsky  and  Schulgin  (’86),  Locy  (’86), 
Bruce  (’87),  Kishinouye  (’90),  and  Laurie  (’90  and  ’92),  we  may 
gain  the  following  summary  of  the  development  of  the  lung 
books  in  these  forms. 

In  these  forms  the  lungs  develop  in  connection  with  the  ab- 
dominal appendages.  These  appendages  grow  out  for  a short 

1 See  upon  this  Kingsley  (’85,  p.  541,  and  ’92,  p.  60). 

2 Thus  Laurie  says  {l.c.  p.  136);  “The  additional  appendages  of  Limulus  are 
directed  towards  the  tail  as  one  would  expect  abdominal  appendages  to  be.  Now 
if  the  appendage  had  sunk  without  invagination,  one  would  expect  it  to  be  still  di- 
rected towards  the  tail,  unless  there  were  some  very  good  reason  for  its  having 
changed  its  direction.  If,  on  the  contrary,  it  had  become  invaginated  it  would 
naturally  be  directed  in  the  opposite  direction  towards  the  head,  and  this  is  what 
we  find  in  the  scorpion.  The  inpushing  is  from  the  beginning  towards  the  head, 
and  the  aperture  opens  toward  the  tail.” 


226 


KINGSLEY. 


[VOL.  VIII. 


distance  and  then  an  inpushing  takes  place  just  behind  the 
appendage,  the  opening  of  the  invagination,  according  to  Kish- 
inouye  being  away  from  the  median  line.  This  sac,  the  future 
pulmonary  cavity,  continues  to  increase  in  size,  while  the  ap- 
pendage proper  soon  becomes  obsolete.  In  this  way  the  pul- 
monary sac,  with  its  external  opening  or  stigma,  is  developed. 
After  the  pulmonary  sac  is  formed  there  begins  on  its  anterior 
wall,  — i.e.  on  the  continuation  of  the  posterior  wall  of  the 
appendage,  — a series  of  foldings  of  the  ectoderm,  the  lung- 
leaves.  As  the  animal  increases  in  size,  new  lung-leaves  are 
added  at  the  inner  or  proximal  end.  In  short,  as  the  adjacent 
diagrams  show,  the  homologies  between  the  two  types  of  organs 
are  perfect. 

In  / we  have  a condition  which  will  apply  equally  well  to 
the  young  of  either  Limulus  or  Scorpio.  At  the  right  side 
the  appendage  is  just  budded  out,  and  on  the  left  the  sinking 


I 


Diagram  of  the  respiratory  organs  in  {A)  an  Arachnid ; (Z)  in  Limulus,  and 
(Z)  in  an  intermediate  condition.  The  arrows  point  toward  the  head,  the  cross- 
lined  portion  is  the  anterior,  the  black,  the  posterior  surface  of  the  appendage,  the 
dotted  surface  that  part  of  the  ventral  surface  of  the  somite,  which  is  invaginated 
to  form  the  posterior  wall  and  roof  of  the  pulmonary  sac.  bs,  blood  space  ; 
gl,  gill-leaves  ; //,  pulmonary  leaves. 


No.  2.] 


THE  EMBRYOLOGY  OF  LIMULUS. 


227 


in,  behind  it,  has  begun.  In  L we  have  the  modifications  of  /, 
which  result  in  the  formation  of  the  gill-book.  On  the  pos- 
terior face  of  the  appendage  the  gill-leaves  are  budding  out. 
In  A we  have  the  Arachnid  modifications  of  I.  On  the  right 
side  the  post  appendicular  insinking  has  resulted  in  the  forma- 
tion of  the  pulmonary  sac  from  the  anterior  wall  of  which, — and 
which  is  plainly  the  posterior  surface  of  the  appendage  — the 
pulmonary  leaves  are  being  produced  (cf.  Laurie,  ’90,  pi.  XVII, 
Fig.  47).  At  the  left  side  of  the  figure  the  same  conditions 
are  carried  further,  and  the  opening  of  the  pulmonary  sac  is 
now  reduced  to  the  narrow  spiracle.  In  all  the  figures  the 
anterior  surface  of  the  appendage  is  crossed-lined,  the  pos- 
terior is  black,  the  invaginated  portion  of  the  ventral  surface 
dotted,  the  rest  of  the  ventral  surface  is  white.  The  arrow 
points  towards  the  anterior  end  of  the  animal. 

When  the  comparison  is  made  in  this  way  the  similarities,  as 
to  appendages  and  lamellae,  are  seen  to  be  very  close.  When 
viewed  from  the  histological  standpoint  the  resemblances  are 
so  exact  that  the  description  of  the  pulmonary  organ  of  the 
spider  or  of  the  scorpion  will  apply  almost,  word  for  word,  as 
shown  above,  to  the  gill  book  of  Limulus.  This,  taken  in  con- 
nection with  the  fact  that  the  very  appendages  which  in  the 
scorpion  (IX-XII)  are  converted  into  the  lung-books,  are  in 
the  Limulus  the  bearers  of  gill-books,  and  that  appendages 
VIII  of  the  scorpion  (the  pectines)  have  a structure  also  easily 
reducible  to  the  gill-book  of  the  corresponding  somite  of  the 
horse-shoe  crab,  place  the  homologies  of  the  organs  in  such  a 
light  that  few  identities  of  structure  and  of  phylogeny  are  more 
certain. 

The  consideration  of  the  relations  existing  between  the  lungs 
and  the  tracheae  of  the  Arachnids  will  be  taken  up  later. 

The  Relationships  of  Limulus. 

It  would  seem  hardly  necessary  to  review  in  detail  the 
discussion  of  the  systematic  position  of  Limulus  since  it  has 
been  done  by  almost  every  author  who  has  treated  of  its 
anatomy  or  ontogeny  or  who  has  studied  the  spiders.  Yet 
some  space  must  be  devoted  to  it  because  of  the  new  facts 


228 


KINGSLEY. 


[VOL.  VIII. 


brought  out  by  the  present  investigations  and  especially 
because  some  of  the  arguments  advanced  by  the  advocates 
of  the  arachnidan  affinities  of  the  horse-shoe  crab  do  not 
seem  to  be  understood  by  several  recent  writers. 

Notwithstanding  the  early  suggestion  of  Strauss-Durckheim 
{teste  Lankester)  and  the  later  one  by  the  younger  Van 
Beneden  (’7l)  there  was  no  serious  question  of  the  relation- 
ship supposed  to  exist  between  Limulus  and  the  Crustacea 
until  the  publication  of  Lankester’s  paper  (m)  “ Limulus  an 
Arachnid.”  Previous  to  that  date  there  was  a general  agree- 
ment that  the  Arthropods  were  divisible  into  two  great  classes : 
— Tracheata  and  Branchiata  — the  division  being  based  pri- 
marily upon  the  method  of  respiration  ; and  this  view  was 
greatly  strengthened  by  Moseley’s  discovery  (’74)  of  tracheae 
in  Peripatus,  thus  apparently  providing  for  a line  of  descent 
for  the  Tracheates  without  the  necessity  of  any  close  associa- 
tion between  these  and  the  Crustacea.  The  Arachnids  were 
of  course  included  in  the  Tracheata  for  in  most  of  the  group 
were  found  tracheae,  apparently  built  upon  the  same  plan  as 
those  of  the  Hexapods,  while  Leuckart  had  shown  long  ago  (’49) 
that  the  pulmonary  sacs  of  the  spiders  and  scorpions  were 
clearly  homologous  with  the  tracheae  of  the  other  Arachnids. 

Although  not  primarily  based  upon  the  respiratory  system 
Lankester’s  conclusions  were  in  substance  that  the  lungs  of 
the  Arachnids  were  homologous  with  the  gills  of  Limulus  ; 
and  the  deduction  necessarily  followed  that  all  Tracheates 
must  have  come  from  a Limuloid  ancestor  or  that  the  group 
“Tracheata”  must  be  polyphyletic  in  origin  and  that  the 
similarities  of  the  tracheae  in  Hexapods  and  Arachnida  must 
be  due  to  homoplassy  rather  than  to  community  of  descent. 

Lankester’s  paper  produced  no  little  discussion  and  the 
points  presented  by  the  numerous  papers  upon  the  subject  as 
well  as  those  based  upon  the  present  im^estigations  may  be 
presented  in  categorical  order  as  follows  - 

I.  Limulus  agrees  with  the  Crustacea  and  differs  from  the 
Arachnida  in  : — 

1.  A branchial  respiration. 

2.  The  possession  of  biramous  appendages. 


No.  2.]  THE  EMBRYOLOGY  OF  LI M ULUS.  229 

3.  The  absence  of  Malpighian  tubules. 

4.  The  absence  of  salivary  glands. 

5.  The  absence  of  embryonic  envelopes. 

6.  The  presence  of  compound  eyes. 

II.  Limulus  and  the  Arachnids  agree  in,  and  both  differ 
from  the  other  “Tracheates”  (Hexapoda  and  Myriapoda)  in  : — 

7.  The  primitively  postoral  condition  of  appendage  I and 
its  later  transfer  to  a prestomial  position. 

8.  The  six-jointed  appendages  II-V. 

9.  The  existence  of  a metastoma  (chilaria)  upon  the  sixth 
somite. 

10.  A regional  division  of  the  body  behind  somite  VI. 

11.  The  openings  of  the  genital  ducts  upon  appendage  VII. 

12.  The  subservience  of  appendages  IX  to  XII  (Limulus 
VIII-XII)  to  respiration. 

13.  The  possession  of  a post-anal  spine. 

14.  The  formation  of  deutova. 

15.  The  formation  of  the  entoderm  by  delamination. 

16.  The  early  appearance  of  metamerism  in  the  body. 

17.  The  occasional  later  appearance  of  appendage  I and  its 
somite. 

18.  The  existence  of  a well-marked  coelom  (schizocoele)  ex- 
tending at  first  into  the  appendages. 

19.  The  extension  of  the  coelom  of  somite  I into  the  pre- 
oral region. 

20.  The  presence  of  a chitinous  entosternite. 

21.  The  presence  of  a posterior  artery  from  the  heart. 

22.  The  possession  of  a pair  of  sternal  arteries  passing,  one 
on  either  side  of  the  oesophagus,  to  unite  below  in  — 

23.  A longitudinal  arterial  canal  upon  or  surrounding  the 
nervous  system. 

24.  The  possession  of  blood  colored  blue  by  haemocyanin. 

25.  In  the  possession  of  reticulate  genital  ducts. 

26.  In  the  method  of  oviginesis. 

27.  In  the  possession  of  nephridia  in  somite  V. 

28.  In  the  pitted  origin  of  the  nervous  system. 

29.  In  the  concentration  of  the  postoral  ganglia  in  a cir- 
cumoesophageal  nerve  ring. 


KINGSLEY. 


[VOL.  VIII. 


230 

30.  The  invaginate  character  of  the  median  eyes. 

31.  The  long  stomodseum. 

32.  The  large  mesenteron. 

33.  The  large  midgut  glands  (hepatopancreas)  emptying  by 
metameric  ducts. 

34.  The  short  proctodseum. 

III.  Limulus  and  the  Arachnids  agree  with  the  Crustacea, 
and  differ  from  the  “Tracheates”  in  points  14,  21,  27,  31,  and 
also  in  : 

35.  The  absence  of  any  differentiated  head. 

36.  The  position  of  the  genital  ducts  in  the  appendages 
near  the  middle  of  the  body. 

37.  The  development  of  the  respiratory  organs  in  connec- 
tion with  the  appendages. 

38.  The  paired  sexual  openings. 

Several  of  the  foregoing  points  need  but  little  discussion 
since  they  have  already  been  considered  both  by  Prof.  Lan- 
kester  (’81)  and  by  myself.  It  is,  however,  to  be  noted  that 
this  enumeration  of  resemblances  and  differences  omits  all 
reference  to  characters  which  are  common  to  all  great  groups 
of  Arthropods,  and  also  to  those  which  are  peculiar  to  any 
one  group,  except  so  far  as  they  are,  apparently,  based  upon 
misconceptions.  It  must  also  be  mentioned  that  Peripatus  is 
omitted  from  the  discussion,  since,  notwithstanding  the  recent 
researches  of  von  Kennel,  Sedgwick,  Sclater  and  Miss  Sheldon, 
its  position  in  the  Arthropod  phylum  is  not  beyond  question. 
So  too  with  the  chilognathous  Myriapods,  since  for  reasons 
which  will  appear  later,  their  relations  to  the  chilopods  are 
exceedingly  doubtful. 

I have  already  discussed  in  the  previous  part  of  this  paper 
the  evidence  presented  by  the  ovigenesis,  in  which  there  is  a 
close  parallel  between  the  Arachnids  and  Limulus,  the  egg  in 
both  passing  into  a follicle  formed  by  the  separation  of  the 
timica  propria  from  the  germinal  epithelium.  I have  also 
considered  the  matter  of  the  origin  of  the  entoderm  in  both 
Limulus  and  the  Arachnids  (and  Pycnogonids)  by  delamination, 
and  the  early  segmentation  of  the  body,  closely  parallel  in  both 
groups,  before  the  appearance  of  the  legs.  A farther  point 


No.  2.]  THE  EMBRYOLOGY  OE  LI M ULUS.  23  I 

of  similarity  is  in  the  tendency  toward  a late  appearance  of 
appendage  I in  both  groups,  it  having  been  noticed  by  both 
Metschnikoff  and  Laurie  in  the  scorpion  and  by  Birula  (’92)  in 
Galeodes.  This,  however,  has  less  weight  than  it  otherwise 
would  have  were  it  confined  to  these  forms  alone,  Grobben 
having  noticed  (’79)  a similar  delay  in  the  appearance  of  the 
antennulae  in  Moina. 

To  several  other  points  exceptions  may  be  adduced.  Thus  a 
chitinous  entosternite  has  been  noticed  in  several  Crustacea, 
e.g.  in  Apus  and  by  Claus  (’92)  in  Ostracodes.  Deutova^  occur 
in  both  Arachnids  and  Limulus  but  as  Zaddach  described  long 
ago  (’4i)  they  are  found  in  Apus  as  well.  In  the  American 
Limulus,  as  in  the  Arachnids,  the  coelom  at  first  extends  into 
the  appendages  (Kishinouye  says  it  does  not  in  the  Japanese 
species)  but  similar  conditions  have  lately  been  shown  to  occur 
in  the  Hexapods.  Reticulate  genital  ducts  occur  in  the  Phyllo- 
pods.  I cannot  agree  with  Kishinouye  that  the  metastoma  of 
Limulus  possesses  a separate  somite,  and  as  I have  already 
pointed  out  (’92,  p.  60)  his  figures  can  receive  another  interpre- 
tation. There  is  no  somite  and  no  neuromere  for  the  metastoma, 
and  as  it  occurs  upon  somite  VI  which  is  already  provided  with 
appendages,  its  appendicular  nature  is  not  apparent.  Meta- 
stomal  structures  occur  in  other  Arthropods;  the  exact  serial 
similarity  between  that  of  Limulus  and  that  of  the  Arachnids 
is  the  important  point. 

The  possession  of  a post-anal  moveable  spine  (telson  in  Limu- 
lus, sting  in  the  scorpions,  multiarticulate  whip  in  Thelyphonus) 
is  not  paralleled  outside  of  these  forms.  It  is  to  be  regarded 
not  as  a somite  or  a series  of  somites  — the  position  of  the 
anus  settles  that  — but  as  an  articulated  outgrowth  of  the  supra- 
anal  region  of  the  terminal  somite  of  the  body. 

The  foregoing  disposes  of  points  9,  13,  14,  15,  16,  17,  20,  25 
and  26,  while  no  discussion  need  here  be  given  to  items  8,  24, 


1 I have  used  this  term,  introduced  by  Claparede,  for  those  molted  cuticula  or 
“ Blastodermhauten  ” which  serve  as  protective  envelopes  — Packard’s  “ vicarious 
chorion  ” — after  the  splitting  of  the  chorion,  and  before  the  young  is  turned  free 
to  shift  for  itself.  Henking  has  called  (’82)  the  same  structures  in  the  Arachnida 
“ apoderma.” 


232 


KINGSLEY. 


[VOL.  VIII. 


29  and  30,  as  they  have  either  been  treated  of  sufficiently  in 
previous  papers  or  they  are  based  upon  conditions  not  described 
in  the  present  series.  For  the  discussion  of  a large  number  of 
the  remaining  points  it  becomes  necessary  first  to  review  briefly 
our  knowledge  of  the  homologies  between  the  somites  in  the 
principal  groups  of  Arthropods. 

Until  we  have  more  evidence  than  we  now  possess  of  the 
total  disappearance  of  a somite  from  the  anterior  portion  of 
the  body  of  any  arthropod  it  will  be  necessary  in  making  our 
comparisons  between  the  regions  in  the  different  groups  to 
proceed  upon  the  assumption  that  the  metamerically  repeated 
portions  are,  somite  for  somite,  the  same  in  the  whole  phylum, 
and  are  to  be  compared  throughout  upon  the  serial  basis,  the 
first  being  equivalent  throughout,  and  so  with  the  second,  and 
so  on,  the  comparison  ceasing  only  with  the  hinder  region  of 
the  body,  where  the  budding  zone  occurs  and  behind  which 
is  the  terminal  or  caudal  lobe.  Upon  no  other  basis  can  any 
comparison  be  made. 

The  great  difficulty  with  this  is  in  the  recognition  of  the 
metameres  in  the  anterior  end  of  the  body  for  there  we  find  a 
tendency  toward  the  obliteration  of  parts  and  the  obsolescence 
of  those  features  by  which  the  existence  of  the  sOmite  is  made 
most  apparent.  The  nervous  system  seems,  at  present,  to 
afford  us  the  most  certain  means  of  recognition  of  the  somites 
and  upon  this  we  must  place  the  most  dependence,  since  in 
some  cases  the  coelom  of  the  somite  may  disappear,  its  meso- 
derm becoming  fused  with  that  of  its  neighbors  while  the 
appendages  may  totally  fail  to  develop.  In  the  Hexapods 
various  authors  have  expressed  the  idea  that  the  so-called  brain 
was  a compound  structure,  and  of  these  the  later  writers  — 
Tichomiroff  {teste  Cholodkowsky)  Patten  (’88)  and  Cholod- 
kowsky  (’9i)  represent  it  as  composed  of  three  neuromeres,  and 
Carriere  (in  Chalicoderma)  has  recognized  still  another  in  the 
cephalic  region.  Of  these  the  most  anterior,  the  protocere- 
brum, is  apparently  prestomial  and  hence  is  to  be  regarded  as 
homologous  with  the  annelid  cerebrum,  and  the  descendant  of 
the  “ Scheitelplatte.”  All  recent  observers  — Patten,  Heider, 
/Wheeler,  Graber,  Carriere,  Cholodkowsky^/.  als. — have  amply 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


233 


confirmed  the  early  observation  of  Weismann  (’63)  that  the 
Hexapod  antennae  are  postoral,  and  have  shown  that  they  are 
innervated  from  the  second  or  deutocerebral  neuromere. 
Between  this  and  the  mandibular  ganglion  is  the  third  com- 
ponent of  the  brain,  the  tritocerebrum,  and  in  a few  forms  this 
has  been  shown  to  have  a small  embryonic  appendage  which 
apparently  becomes  obsolete  in  the  later  stages.  This  has 
been  observed  by  Tichomiroff  {teste  Cholodkowsky  in  the 
silk- worm  ; by  Carriere  (’9o)  in  the  bee  Chalicoderma^  while  in 
a note  Dr.  Wheeler  sends  me  an  account  and  a drawing  of  the 
embryo  of  the  Collembolan,  Anurida  maritima,  in  which  the 
appendage  between  the  antenna  and  the  mandible,  the  trito- 
cerebral  appendage,  is  well  marked.  Apparently  Wheeler  had 
seen  traces  of  the  same  in  Doryphora  (’89,  p.  337  Fig.  44). 
Regarding  these  neuromeres  and  appendages  the  conclusion  is 
inevitable  that  they  belong  to  the  primitively  postoral  series. 
They  arise  in  the  same  line  and  agree  in  all  respects  with  those 
further  back.  The  only  other  supposition  would  be  that  they 
are  preoral  and  temporarily  wander  backwards  to  be  immedi- 
ately returned  to  their  proper  position,  a supposition  of  very 
doubtful  value.  In  the  light  of  these  observations  we  must 
regard  the  Hexapod  head  as  composed  of  at  least  six  elements, 
the  procephalic  lobes  and  five  postoral  somites,  each  of  the 
latter  having  appendages. 

In  the  Arachnida  our  evidence  is  much  less  abundant  and 
much  less  detailed.  Schimkewitsch  (’89)  describes  in  the  spiders 
ocular  and  rostral  ganglia  (PI.  XXI,  Fig.  3)  in  advance  of  the 
ganglion  of  the  chelicerae,  while  in  the  schematic  Fig.  5 of  his 
PI.  XXI II  he  represents  the  cerebral  ganglion  (in  front  of  the 
rostral  ganglion)  as  three-lobed.  Patten  (’90)  describes  the 
brain  of  the  scorpion  as  composed  of  three  pairs  of  ganglia,^ 
while  Jawonowsky  (’92)  figures  four  postoral  somites  in  front  of 
the  cheliceral  somite,  the  posterior  of  which  bears  a pair  of 

1 In  his  preliminary  paper  Tichomiroff  merely  says  “Es  existirt  bei  dem  Seide- 
wurm  eine  echte  untere  Lippe  . . . und  die  als  der  allbekannten  Oberlippe  der 
Insecten  homolog  betrachtet  werden  darf.”  His  later  paper  is  unfortunately 
buried  in  an  outlandish  tongue. 

2 Carriere  recognizes  four  cerebral  somites  and  finds  a preantennal  appendage. 

3 See  also  Metschnikoff  (’70),  PI.  XV,  Fig.  14. 


KINGSLEY. 


[VOL.  VIII. 


234 

appendages,  possibly  the  same  as  that  of  Croneberg  (’80)  in 
Dendryphantes.  No  preoral  ganglion  is  indicated  in  his  ex- 
tremely unsatisfactory  figures,  while  his  evident  desire  to  find 
the  Hexapod  antennae  in  the  Arachnids  has  possibly  influenced 
his  observations.^  Locy  has  also  figured  (’86,  PI.  XI,  Fig.  70) 
a distinctly  three-lobed  brain  in  Agelena.  I regret  that  I have 
not  been  able  to  consult  the  original  plates  of  Morin’s  account 
of  the  development  of  the  spiders,  but  as  copied  by  Korshelt 
and  Heider  (’92,  Fig.  383B)  the  brain  of  Theridium  consists  of 
four  lobes,  the  posterior  of  which  is  apparently  the  cheliceral 
ganglion.  Kishinouye  (’90)  also  describes  the  brain  of  Agelaena 
as  three-segmented. 

In  Limulus  both  Patten  and  myself  have  recognized  a three- 
ganglioned  cerebrum  in  front  of  the  ganglia  of  the  chelicerae. 
None  of  these  cerebral  ganglia  have  been  seen  by  me  in  a 
postoral  position,  but  their  relation  to  the  ventral  chain  is  such 
as  to  justify  the  supposition  that  here,  as  in  the  Hexapod,  there 
is  a very  early  shifting. 

In  the  Crustacea  I know  of  no  observations  of  evanescent 
appendages  or  neuromeres,  unless,  possibly  in  the  case  of  the 
metastoma.  So  far  as  observations  go  the  series  is,  appar- 
ently: first,  the  procephalic  lobes  ; second,  a pair  of  ganglia  in 
front  of  the  antennulae,  figured  by  Bumpus  (’91,  PI.  XVII,  Fig. 
i)  then,  antennulae,  antennae,  etc.  As  to  just  where  the  line 
between  preoral  and  postoral  is  to  be  drawn  is  uncertain.  That 
the  antennulae  of  the  Crustacea  are  to  be  classed  in  the  primi- 
tively postoral  series  is  evidenced  by  several  facts.  In  the  first 
place,  they  are  placed  by  all  observers  at  first  in  a paraoral  if 
not  a postoral  position,  and  in  a direct  continuation  of  the  post- 
oral appendages.  Secondly,  the  evidence  presented  by  Apus  is 
clearly  understood  upon  the  basis  of  a complete  transfer  of  the 
ganglia  and  appendages  to  a preoral  position.  In  the  adult  the 
ganglia  of  the  antennulae  {cf.  Pelseneer,  ’85)  are  fused  with  the 
cerebrum,  while  the  course  of  the  nerves  (see  Zaddach,  ’41,  PI. 
HI,  Figs.  I and  5)  shows  distinctly  a transfer  of  the  structures 

1 In  the  light  of  the  observations  of  Carriere  and  Wheeler  his  discovery  does 
not  help  matters,  for  there  is  still  a somite  lacking,  to  make  the  parallel  exact 
from  his  standpoint. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


235 


at  either  end  of  the  cord.  I do  not  quote  my  own  observations 
upon  the  postoral  position  of  the  antennulae  of  Crangon  because 
their  accuracy  has  lately  been  denied  by  Weldon^  (’92)  and  by 
Herrick  (’92),  and  hence  they  need  confirmation. 

It  is  evident,  I think,  from  the  foregoing  resume  that  we 
cannot  with  much  confidence  compare,  somite  for  somite,  the 
bodies  of  even  the  most  studied  Arthropods,  but  we  may 
present  the  following  tentative  statement,  merely  remarking 
that  as  far  as  Arachnid  and  Limulus  are  concerned,  the  cor- 
rectness of  our  assumption  receives  confirmation  from  other 
sources  discussed  in  this  essay. 


Hexapod. 

Arachnid. 

Limulus. 

Crustacea. 

Neuromere  I 

No  Appendage 

No  Appendage 

No  Appendage 

No  Appendage 

“ II 

Antennae 

No  Appendage 

No  Appendage 

No  Appendage 

“ III 

Apjoendage 

No  Appendage 

No  Appendage 

Antennula 

“ IV 

Mandible 

Chelicera 

1st  Leg 

Antenna 

» V 

Maxilla 

Pedipalpus 

2d  Leg 

Mandible 

“ VI 

Labium 

1st  Leg 

3d  Leg 

Maxilla  i 

“ VII 

I St  Leg 

2d  Leg 

4th  Leg 

Maxilla  2 

“ VIII 

2d  Leg 

3d  Leg 

5th  Leg 

Maxilliped  i 

“ IX 

3d  Leg 

4th  Leg 

6th  Leg 

Maxilliped  2 

It  will  be  noted  that  this  comparison  brings  the  end  of  the 
Hexapod  thorax,  and  the  hinder  margin  of  the  cephalothorax 
of  both  the  Arachnids  and  Xiphosures  into  correspondence. 
Further,  if  we  insert  into  the  Crustacean  line  a segment  for 

1 Professor  Weldon  and  myself  are  apparently  at  variance  upon  several  points 
with  regard  to  the  embryology  of  Crangon,  but  the  points  in  dispute  cannot  be 
settled  except  by  renewed  observation.  I would,  however,  point  out  that  in 
several  places  he  has  attributed  to  me  views  which  I do  not  hold  and  which  he 
would  not  have  obtained  had  he  read  my  papers  carefully  or  had  he  availed  him- 
self of  his  opportunity  to  talk  over  the  points  of  difference  while  his  paper  was  in 
press.  Thus  he  says  (/.  c.  p.  349)  that  I claim  that  the  blastopore  closes  com- 
pletely. I have  (’89,  p.  6)  repudiated  this  view.  He  refers  to  my  “ remarkable 
Fig.  32  ....  in  which  a black  dot  placed  between  the  optic  lobes  is  called  the 
mouth  ” as  representing  my  evidence  as  to  the  postoral  nature  of  the  antennulas. 
The  figure  is  distinctly  stated  to  be  a diagram  to  illustrate  the  plane  of  the  sec- 
tions ; Fig.  1 1 is  the  one  to  which  he  should  have  referred.  I have  no  desire  for 
controversy,  but  would  respectfully  suggest  that  possibly  Weldon’s  figure  (7), 


236 


KINGSLEY. 


[VOL.  VIII. 


the  metastoma  (which  numerous  authors  have  regarded  as  an 
appendage  homodynamous  with  the  others)  the  result  will  be 
to  bring  the  third  thoracic  foot  of  the  Hexapod  into  homology 
with  the  first  rather  than  the  second  maxilliped  of  the  Crus- 
tacea, and  there  is  no  little  evidence  to  show  that  here,  if 
anywhere,  the  line  between  head  and  thorax  in  the  Crustacea 
is  to  be  drawn.  We  are,  however,  more  concerned  at  present 
with  the  serial  comparison  between  Limulus  and  the  Arachnids, 
and  the  studies  of  the  nervous  system  warrant  the  comparisons 
made  above. 

Although  we  are  fully  justified  in  the  recognition  of  somites 
in  both  Limulus  and  Arachnids  in  front  of  the  segment  of  the 
first  appendage,  I have,  from  convenience,  followed  the  old 
nomenclature  in  the  following  discussion,  and  have  numbered 
the  somites  according  to  the  appendages,  somite  I being  that 
of  the  first  appendage.  From  the  foregoing  it  will  be  seen 
that  point  7,  the  transfer  of  appendage  I from  a postoral  to  a 
prestomial  position  has  but  little  value  in  deciding  the  affini- 
ties of  Limulus,  while,  if  our  comparisons  be  correct,  point 
10,  the  existence  of  a regional  division  of  the  body  behind 
appendage  VI  occurs  in  the  Hexapods  and  possibly  in  the 
Crustacea  as  well.  Point  35 — the  absence  of  any  differen- 
tiated head  — is  closely  allied  to  this  last.  In  the  Arthropods 
the  terms  “head”  and  “thorax”  must  be  used  with  physio- 
logical rather  than  morphological  values.  The  cephalothorax 
of  Limulus,  as  well  as  that  of  the  Arachnids,  is  co-extensive,  so 
far  as  our  present  knowledge  goes,  with  both  head  and  thorax 


upon  which  he  relies  to  support  his  statement  that  “the  first  antennae  are 
evidently  preoral  from  the  very  earliest  period  at  which  the  mouth  is  visible,” 
represents  not  the  first  antennae,  but  the  optic  lobes  alone.  Herrick  says  (’92,  p. 
442)  that  he  cannot  agree  with  me  in  saying  that  Reichenbach  “has  all  the 
appendages  at  first  distinctly  postoral.”  Reichenbach  figures  (Fig.  ^a)  the  condi- 
tion to  which  I referred.  A leader  {lb.)  goes  to  the  “labrum,”  a thickening  of 
cells  some  distance  in  front  of  his  antennulae  (A.ii).  His  sections  show  that  there 
is  behind  this  but  (if  I read  his  description  aright)  still  in  front  of  the  atenn- 
nulae,  a mass  of  cells,  his  “ Vorderdarmkeim.”  If  this  be  so  I am  certainly  justi- 
fied in  my  reference  to  Reichenbach  as  showing  the  mouth  in  front  of  the  first 
pair  of  appendages.  There  is  as  yet  no  functional  mouth  found,  but  the  collection 
of  cells  indicated  by  Reichenbach  marks  the  point  of  the  later  stomodaeal  invagi- 
nation. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


237 


of  the  Hexapod,  and — if  we  be  permitted  to  recognize  a 
metastomal  somite  in  the  Crustacea  — with  the  ‘‘head”  of  the 
Tetradecapods  and  also  of  that  of  the  Decapods  as  limited  by 
Milne  Edwards,  but  not  with  the  Decapod  head  as  understood 
by  Huxley.  The  argument  which  Huxley  draws  for  placing 
the  division  between  head  and  thorax  in  the  Crustacea  between 
appendages  V and  VI,  is  largely  based  upon  his  views  of  the 
cervical  suture  of  the  crayfish  which  Dana  long  before  (’5i) 
showed  to  be  untenable  and  which  Ayers  (’85)  has  more  lately 
reviewed. 

Upon  the  standpoint  we  have  taken  the  cephalothorax  of 
the  Arachnids  and  Limulus  must  be  regarded  as  equivalent 
to  the  combined  head  and  thorax  of  the  Hexapod.  In  the 
forms  first  mentioned  we  find  no  tendency  towards  a differ- 
entiation of  this  region  except  in  the  case  of  the  Solpugids, 
a knowledge  of  whose  embryology  would  prove  so  interesting. 
As  the  Solpugids  are  not  primitive  forms,  and  as  no  such 
regional  divisions  occur  in  the  more  ancestral  types,  we  would 
rather  suspect  that  the  apparent  existence  of  the  Hexapod 
thorax  in  the  group  was  secondary  and  adaptional  rather  than 
derived  from  a common  ancestor.  Thorell’s  view  that  the 
Solpugids  are  Hexapods  is  not  tenable. 

In  this  connection  the  multiarticulate  character  of  the  an- 
terior appendages  of  both  Limulus  and  the  Arachnids  is  inter- 
esting. In  the  Hexapods  and  Myriapods  the  mandibles  are 
at  no  time  of  either  embryonic  or  adult  life  multiarticulate, 
a fact  which  would  apparently  indicate  that  this  appendage 
had  obtained  its  present  form  and  function  at  an  early  period. 
It  is,  as  Lang  has  suggested,  hardly  to  be  supposed  that  the 
well  segmented  corresponding  appendage  of  the  Arachnids 
has  been  derived  from  the  specialized  mandible  of  the  Hexa- 
pods. The  modification  of  the  basal  joints  (coxa)  of  several 
appendages  in  both  Limulus  and  scorpions  for  manducatory 
purposes  should  be  alluded  to  here  as  well  as  the  persistence 
of  the  same  number  — six  — of  articles  in  the  legs  of  these 
animals. 

It  would  hardly  seem  necessary  to  review  in  detail  the 
arguments  for  the  homology  of  the  respiratory  organs  of 


KINGSLEY. 


[VOL.  VIII. 


238 

Limulus  and  the  Arachnids  were  it  not  that  the  question  is 
frequently  misunderstood.  The  difficulty,  at  least  in  some 
instances,  seems  to  lie  in  the  failure  to  recognize  the  possi- 
bility of  the  tracheae  of  the  Hexapods  and  those  of  the  Arach- 
nids being  homoplastic  rather  than  homologous  organs.  Thus 
to  these  persons  the  attempt  to  derive  the  tracheae  of  the 
Arachnid  from  the  gill  of  some  branchiate  form  seems  to 
imply  the  promotion  of  the  Arachnids  to  the  position  of 
the  “Stammform”  of  the  Tracheata,  a conclusion  which  no 
one  would  care  to  defend  at  the  present  day. 

There  exists  at  present  no  question  of  the  accuracy  of  the 
view  of  Leuckart  (’49)  that  the  lungs  of  the  scorpions  and 
spiders  on  the  one  hand  and  the  tracheae  of  the  Araneina 
and  other  Arachnids  are  homologous,  but  these  organs  differ 
in  one  important  respect  from  the  tracheae  of  the  Hexapods 
which  would  prevent  their  close  comparison.  In  the  Hexapods 
(as  also  in  the  Chilopodous  Myriapoda)  the  stigmata  are 
placed  outside  or  dorsal  to  the  appendages  and  they  never 
develop  in  connection  with  the  legs.  The  observations  of 
Chun  (’75)  followed  by  those  of  later  writers  would  tend  to 
show  that  the  Hexapod  tracheae  were  derived  from  dermal  ^ 
glands.  In  the  Arachnids,  observations  are  as  yet  lacking  as 
to  the  ontogeny  of  the  tracheae,  but  several  students  have 
described  the  development  of  their  homologues,  the  pulmonary 
organs. 

The  lungs  of  scorpions  — Metschnikoff,  (’7i);  Kowalevsky 
and  Schulgin  (’86)  Laurie  (’9o)  — and  those  of  spiders  — Bruce 
(’87) ; Locy  (’86)  — develop  in  connection  with  the  abdominal 
feet  in  the  embryo.  The  lung-leaves  arise  as  outgrowths  upon 
the  posterior  faces  of  these  appendages,  concomitantly  with  the 
formation  of  a pit  behind  the  appendage  and  the  sinking  of  the 
appendage  itself.  In  this  it  is,  making  allowance  for  the  position 
of  the  organ  — freely  projecting  in  the  one,  sunken  in  the  other 
— closely  comparable  to  the  gills  of  Limulus,  and  in  the  later 
stages,  the  resemblance  extends  to  the  minute  histological  details. 

1 The  recent  speculations  of  Bernard  (’92,  ’93)  in  which  the  endeavor  is  made 
to  trace  all  tracheae  — Hexapod  and  Arachnid  — to  the  setiparous  glands  of  the 
Chaetopods  should  possibly  be  referred  to  here. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


239 


Between  the  lungs  of  the  scorpion  and  the  gills  of  Limulus 
the  resemblances  are  closest.  In  Limulus  the  gills  are  borne 
on  appendages  VIII-XII,  in  the  scorpion  upon  appendages 
IX-XII  and  in  no  Arachnid  do  tracheae  occur  behind  this  point. ^ 
Farther,  appendage  VIII  in  the  scorpion  — the  pecten  — shows 
plainly  its  homologies  with  its  homologue  in  Limulus,  the  teeth 
of  the  comb  being  the  gill-leaves.  This  is  exactly  what  we 
should  expect  upon  our  hypothesis,  for  the  scorpions,  where  the 
resemblances  are  closest,  are  admitted  by  all  to  be  the  most 
primitive  of  the  Arachnids  and  which  naturally  should  possess 
the  most  ancestral  type  of  respiratory  organs.  The  other  view, 
that  the  lungs  are  modified  tracheae,^  leads  into  considerable 
difficulties  for  we  then  find  the  oldest  stock, — the  Stamm- 
form  of  the  Arachnida  — possessing  the  most  highly  differ- 
entiated organs  of  breathing,  while  in  the  most  aberrant  groups 
the  tracheae  have  been  retained  in  an  unmodified  condition. 
Again  the  Arachnida  as  a class,  according  to  the  observations 
of  Plateau  (’86)  and  Berteaux  (’90),  show  an  entire  absence  of 
those  visible  respiratory  movements  of  the  body  wall  which  are 
so  characteristic  of  Hexapods  and  Chilopods,  a fact  in  full 
accordance  with  the  thesis  here  maintained  but  not  easily  ex- 
plained upon  the  standpoint  of  a common  origin  of  all  Arthropod 
tracheae. 

Farther,  the  conversion  of  the  gills  directly  into  tracheal 
tubes  is  at  present  going  on  in  the  case  of  the  Oniscid  Crustacea 
where  we  have  tubes  lined  with  a chitinous  intima  penetrating 
to  the  interior  of  the  organ  and  conveying  air  to  the  blood. 

1 Bernard  claims  (’93)  to  have  found  traces  of  stigmata  in  the  Pseudoscorpions 
behind  this  point,  but  apparently  his  discovery  is  not  a new  one  for  von  Siebold 
pointed  out,  over  forty  years  ago  (’53,  p.  370)  that  Bernard’s  predecessors  had 
also  mistaken  the  cutaneous  insertion  of  muscles  for  stigmata. 

2 This  view  is  held  by  Sinclair  (’92)  who  seems  to  ignore  the  possibility  of  there 
being  two  kinds  of  tracheae;  and,  influenced  by  his  observations  upon  the  peculiar 
dorsal  tracheae  of  Scutigera,  states  his  opinion  “ that  we  have  a series  from  the 
simple  tracheae  found  in  Peripatus  up  to  the  complete  lungs  of  spiders  which  is 
incapable  of  explanation  in  the  present  state  of  our  knowledge,  except  as  repre- 
senting the  stages  of  development  of  tracheae  into  the  pulmonary  organ  of  spiders.” 
A few  lines  lower  he  seems  to  think  that  the  derivation  of  the  lungs  of  scorpions 
from  gills  implies  a difference  between  spiders  and  scorpions  greater  than  has  been 
supposed. 


240 


KINGSLEY, 


[VOL.  VIII. 


Cases  like  this  clearly  show  us  that  tracheae  may  arise  in 
different  ways  from  different  sources. 

The  presence  of  the  so-called  spiral  thread  in  the  tracheae  of 
both  Arachnids  and  Hexapods  has  been  adduced  as  an  argument 
in  favor  of  the  homology  of  the  organs  in  both  groups.  As  in 
both  cases  the  tracheae  are  formed  as  invaginations  of  the 
external  integument  it  is  natural  that  they  should  consist  of 
tubules  of  ectoderm  lined  with  a chitinous  intima,  and  the 
thinner  this  intima  the  easier  the  transfer  of  gases  through  it. 
But  if  it  become  too  thin  the  tube  is  liable  to  total  collapse  by 
the  pressure  of  the  various  viscera  upon  it  and  so  the  chitinous 
layer  is  developed  into  folds  or  corrugations,  which  when  regu- 
larly arranged  form  the  spiral  “threads.”  In  many  spiders  they 
are  not  regular  and  show  clearly  their  origin  in  response  to  the 
mechanical  conditions  presented. 

The  greatest  difficulty  in  connection  with  this  view  of  the 
origin  of  tracheae  from  gills  through  the  lungs  is  that  presented 
by  the  Solpugids  and  certain  Acarina  where  tracheal  stigmata 
occur  in  the  cephalothoracic  region,  where  they  should  not 
occur  according  to  our  thesis.  Yet  until  we  know  more  of 
the  structure  and  ontogeny  of  these  organs  the  full  weight 
of  this  objection  cannot  be  properly  estimated.  A full  his- 
tory of  Solpuga  would  settle  many  questions  of  arthropod 
morphology. 

To  summarize : The  lungs  of  the  scorpion  arise  in  the  same 
way  and  on  the  same  somites  as  the  gills  of  Limulus.  In  the 
one  they  sink  into  a pit,  in  the  other  they  remain -free.  The 
homologies  between  the  lungs  and  tracheae  of  Arachnida  were 
demonstrated  by  Leuckart.  Hence  the  lungs,  the  fan  tracheae 
of  authors,  are  to  be  regarded  as  the  primitive,  the  bush-like 
the  derived  form,  and  these  tracheae  have  no  relation  to  those 
of  Hexapods. 

The  comparisons  between  the  nephridia  of  Limulus  and 
those  of  the  Arachnida  have  been  made  upon  a previous  page. 
The  observations  made  by  Laurie,  Kishinouye,  and  myself  have 
clearly  shown  that  we  have  to  deal  here  with  structures  homol- 
ogous with  the  nephridia  of  the  worms,  although  but  a single 
pair  may  persist  in  its  unmodified  condition.  We  find,  how- 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


241 


ever,  nephridia  occurring  either  in  the  young  or  the  adult  of 
other  Arthropods,  and  hence  a more  accurate  review  of  our 
knowledge  becomes  necessary.  In  Peripatus  the  investigations 
of  von  Kennell  (’84),  and  especially  of  Sedgwick  (’88),  have 
shown  that  in  each  somite,  except  the  posterior  one,  the  coelom 
on  either  side  divides  into  dorsal,  lateral  and  ventral  moities; 
the  dorsal  becomes  converted  into  the  gonad,  while  the  ventral 
portion  becomes  converted  one  part  into  the  nephridium  and  the 
lateral  into  the  funnel  and  end  sac.  The  connection  between 
the  dorsal  and  ventral  portions  of  the  coelom  persists  in  the 
posterior  somite,  and  from  the  cavity  thus  formed  the  genital 
ducts  are  developed,  in  other  words,  the  posterior  nephridia 
of  Peripatus  become  modified  for  reproductive  ducts.  In 
the  Hexapods  Heymons  (’90),  Graber  and  Cholodkowsky  (’9i)^ 
have  described  a similar  division  of  the  coelom  into  three  por- 
tions, the  gonad  developing  in  connection  with  the  dorsal 
portion;  the  formation  of  genital  ducts,  much  as  in  Peripatus; 
and  the  development  of  the  third  division  into  a temporary 
structure  to  be  regarded  as  the  homologue  of  the  nephridium 
of  Peripatus,  and  which  later  disappears.  In  the  Crustacea 
nephridial  structures  also  occur.  Our  knowledge  of  them  and 
of  their  relations  to  the  coelom  are  most  detailed  in  Decapod. 
Here  Weldon  (’89,  ’9i)  has  described  a large  coelomic  dorsal 
sac,  extending  back  to  the  heart  and  the  gonads  and  connected 
ventrally  with  the  green  (antennal)  gland,  the  character  of 
which  as  a nephridium  is  thus  placed  beyond  question.^  The 
position  of* the  so-called  “shell  gland”  is  less  certain,  though 
all  evidence  goes  to  show  that  this  is  also  to  be  regarded  neph- 
ridial. As  I pointed  out  several  years  ago  this  organ,  opening 
in  the  Crustacea  at  the  base  of  the  second  maxilla  is  apparently 
exactly  homologous  with  the  coxal  gland  of  Limulus.  Al- 
though recent  researches  {vide  supra)  have  changed  our  views 

1 There  is  not  full  agreement  between  these  authors  as  to  the  details  of  the 
process. 

2 Both  Grobben  (’79)  and  myself  (’89)  have  shown  that  the  green  gland  of  the 
decapod  is  mesodermal.  Richenbach  in  his  first  paper  upon  Astacus  (’77)  stated 
that  it  was  derived  from  the  ectoderm.  Although  he  was  corrected  in  this  by 
Grobben  he  reiterates  his  account  in  his  later  paper  on  the  crayfish  (’86)  and 
ignores  Grobben's  correction. 


242 


KINGSLEY. 


[VOL.  VIII. 


of  the  somites  of  the  Arthropods,  still  if  the  metastoma  be  ad- 
mitted as  an  appendage  in  the  Crustacea,  the  correspondence 
between  the  opening  of  the  duct  of  their  shell  gland  and  that 
of  the  coxal  gland  is  exact.  Another  fact  which  goes  to  show 
that  the  shell  gland  is  nephridial  is,  that  it  and  the  antennae 
gland  but  rarely  coexist  in  the  same  individual  (Nebalia,  Claus). 

From  the  evidence  presented  by  the  nephridia  therefore,  we 
are  justified  in  the  close  association  of  the  Arachnids  and  the 
Xiphosures.  We  are  also  apparently  led  to  associate  these  two 
groups  more  closely  with  the  Crustacea  than  with  the  Hex- 
apods. 

The  correspondence  between  the  genital  ducts  of  Limulus 
and  those  of  the  Scorpion  is  as  close  as  that  of  the  respira- 
tory organs.  In  Limulus  the  genital  ducts  in  both  sexes  open 
upon  the  posterior  surface  of  appendage  VII.  In  the  scorpions 
Narayanan  (’89)  has  shown  that  the  genital  operculum  is  a 
paired  organ  in  both  sexes  and  that  the  genital  ducts  open 
up  on  what  is  morphologically  its  posterior  surface.  Farther, 
Laurie’s  observations  upon  the  development  of  the  ducts  show 
beyond  a question  that  they  belong  to  somite  VII.  In  Limu- 
lus I have  failed  to  see  the  development  of  the  duct,  it  being 
apparently  delayed  even  longer  than  in  the  scorpion. 

There  is  to-day  little  doubt  that  the  genital  ducts  of  all 
Arthropods  are  to  be  regarded  as  modified  nephridia.  I have 
alluded,  just  above  to  the  method  of  development  of  these 
structures  in  the  Hexapods  and  in  Peripatus.  In  the  scorpion 
Laurie’s  account  of  their  development  would  indicate  that  here, 
too,  they  are  to  be  classed  in  the  same  category,  the  later 
appearance  of  their  external  opening  being  the  greatest  objec- 
tion to  such  a view. 

In  the  Crustacea  we  have,  so  far  as  I am  aware,  no 
direct  observations  upon  the  ontogeny  of  the  ducts,  but  the 
facts  of  comparative  anatomy  are  all  but  conclusive.  Thus  the 
relations  of  the  gonads  to  the  persistent  coelom  are  such  as 
would  be  required  were  the  ducts  segmental  organs,  while  the 
varying  position  of  the  ducts  themselves  in  the  two  sexes  of 
the  same  species  and  the  fact  that  in  abnormal  instances  two 
pairs  of  ducts  may  occur  in  the  same  individual,  show  that  they 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  243 

must  have  been  derived  from  some  metameric  structure 
connecting  the  coelom  with  the  exterior,  and  the  nephridia  of 
the  annelids  are  the  most  probable  if  not  the  only  ducts  which 
answer  the  conditions. 

Limulus,  the  Arachnids,  the  Crustacea  and  the  Chilognaths 
agree  in  having  the  genital  ducts  some  little  distance  in 
advance  of  the  posterior  end  of  the  body  while  in  the  Hexa- 
pods and  Chilopods  they  are  sub-terminal,  but  how  much 
weight  is  to  be  given  this  point  is  not  yet  apparent. 

The  reticulate  and  anastomosing  character  of  the  genital 
ducts  in  Limulus  and  the  Arachnids  has  been  commented  upon 
by  Lankester.  Such  conditions  are  not  paraded  in  the  Ar- 
thropods except  in  certain  Phyllopods.  Again  the  existence  of 
motile  spermatozoa  in  both  Limulus  and  Arachnids  and  their 
absence  from  all  Crustacea  except  the  Cirripedia  has  a certain 
value  as  cumulative  evidence. 

The  correspondences  between  the  circulatory  systems  of 
Limulus  and  the  scorpions  are  remarkably  close.  In  both 
there  is  the  same  median  anterior  aorta  which  divides  and 
passes  downward,  as  a pair  of  sternal  arteries  — one  passing 
on  either  side  of  the  oesophagus  — which  unite  below  in  a 
ventral  vessel  in  close  connection  with  the  ventral  nerve  chain. 
In  the  scorpions  this  ventral  vessel  consists  of  an  artery  ^ 
lying  ttpon  the  nervous  system,  and  following  not  only  the 
ventral  cord  but  the  various  metameric  nerves  which  arise 
from  it.  This  condition,  which  is  characteristic  of  the  adult 
scorpion  is  found  in  the  earlier  stages  of  Limulus.  Later  the 
neural  artery  completely  envelopes  the  ventral  cord  and  its 
nerves  in  the  manner  first  pointed  out  by  Owen  (’55,  p.  310) 
and  later  so  elaborately  described  by  the  younger  Milne- 
Edwards  (’72). 

This  relation  between  the  neural  artery  and  the  ventral 
nerve  chain  is  not  confined  to  the  Arachnids  and  Limulus  ; a 
large  supra-neural  vessel  occurs  in  the  Isopods  and  a smaller 
one  in  the  Amphipods,  each  connected  with  the  dorsal  vessel 

1 Houssay  (’87)  claims  that  this  vessel  in  the  scorpion  is  lacunar  rather  than 
arterial,  a view  which  is  negatived  by  its  well-marked  walls  and  its  lack  of  connec- 
tion with  the  other  extra-vascular  spaces  of  the  body. 


244 


KINGSLEY. 


[VOL.  VIII. 


by  similar  paired  sternal  arteries.  A similar  supra-neural 
vessel  was  described  long  ago  by  Newport  (’43)  in  several 
Myriapods.  The  supra-neural  vessel  of  the  Chaetopods  will 
naturally  suggest  itself  in  this  connection.  In  the  Hexapods, 
on  the  other  hand,  the  sternal  arteries  and  the  neural  artery 
have  disappeared,  possibly  as  a result  of  their  richly  developed 
tracheal  system.  In  Peripatus  also  no  supra-neural  vessel  is 
found,  the  ventral  vessel  first  described  by  Balfour  (’83)  lying 
in  the  body  wall  and  the  “blood  spaces”  shown  in  Sedg- 
wick’s monograph  lying  near  the  ventral  cord,  are  lacunar 
rather  than  arterial. 

The  alimentary  canal  of  Limulus  and  the  Arachnids  agrees 
in  the  fact  that  nearly  the  whole  tract  is  composed  of  stomo- 
daeum  and  mesenteron  while  the  late  appearing  proctodaeum  is 
short.  They  also  agree  in  the  metameric  nature  of  the  lobula- 
tion of  the  hepatopancreas,  the  lobes  being  at  first  outlined  by 
the  ingrowth  of  the  mesodermic  septa  into  the  yolk.  In  the 
Hexapods  on  the  other  hand  the  proctodaeum  appears  much 
earlier  and  is  comparatively  long,  at  least  equalling  the  stomo- 
daeum  in  this  respect.  In  the  Crustacea  on  the  other  hand  the 
mesenteron  plays  but  an  inconspicuous  part  in  the  formation 
of  the  digestive  tube,  it  being  mostly  restricted  to  the  so-called 
liver. 

The  possession  of  an  entosternite  which  characterizes  both 
Limulus  and  the  Arachnids,  the  structure  and  relationships  of 
which  has  already  been  discussed  by  Lankester  (’84)  is  only 
paralleled  outside  these  forms  in  a few  Crustacea  (certain 
Ostracodes,  Claus,  ’92). 

The  argument  for  the  association  of  Limulus  with  the  Crus- 
tacea and  its  separation  from  the  Arachnids,  based  upon  the 
possession  of  biramous  appendages,  has  been  accorded  more 
weight  than  seems  justifiable.  At  no  stage  of  development  do 
we  find  a biramous  condition  in  the  cephalothoracic  appendages 
of  Limulus,  while  that  of  the  abdominal  appendages  may  prove 
to  be  far  different  from  that  of  the  Crustacea.  It  appears  much 
later  than  in  the  Crustacea,  is  characterized  by  a hypertrophy 
of  the  exopodite,  and  lacks  the  evident  segmentation  found  in 
most  Crustacea. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


245 


On  the  other  hand,  we  must  not  lose  sight  of  the  fact  that 
numerous  observers  have  recorded  a biramous  condition  in  the 
appendages  of  various  “ Tracheates.”  Among  others  we  would 
mention  the  biramous  pedipalps  in  Dendryphantes  recorded  by 
Croneberg  (’80),  the  biflagellate  antenna  of  an  Indian  Lepisma, 
and  of  an  embryo  Blatta  javanica  by  Wood  Mason  (’79),  the 
bifid  condition  of  the  antenna  of  Blatta  by  Wheeler  (’89),  while 
Patten  (’84),  in  the  same  form  describes  the  maxillae  and  labium 
as  “ formed  respectively  of  two  and  three  branches,  the  second 
maxillae  thus  attaining  the  typical  trichotomous  structure  of  the 
Crustacean  appendages.”  Neither  must  we  forget  the  peculiar 
antennae  of  the  Pauropida  in  this  connection. 

The  so-called  Malpighian  tubes  (point  3.)  have  a far  dif- 
ferent bearing  upon  the  classification  of  the  Arthropods  from 
that  which  they  were  supposed  to  have  a few  years  ago.  In 
fact,  two  entirely  different  structures  have  been  included  under 
the  one  name,  and  the  existence  of  excretory  tubules  in  both 
Hexapods  and  Arachnids,  instead  of  proving  the  close  relation- 
ship of  the  two  groups,  is,  in  view  of  our  present  knowledge, 
an  argument  against  it.  In  the  Hexapods  these  organs  have 
been  shown  by  numerous  observers  to  be  of  proctodeal,  and, 
therefore,  of  ectodermal  origin.  In  the  Arachnida  the  supposed 
homologous  organs,  to  which  the  same  name  has  been  given, 
are,  in  all  probability,  outgrowths  from  the  mesenteron,  and 
hence  entodermal.  This  has  been  shown  by  Loman  (’86-7)  for 
both  the  tetra-  and  the  dipneumonous  Araneina,  and  by  Laurie 
(’90)  for  the  scorpion.^  Hence  these  organs,  — ectodermal  in 
the  one  group,  entodermal  in  the  other  — instead  of  indicating 
community  of  descent  for  Arachnids  and  Hexapods,  must  rather 
be  regarded  as  indicating  that  the  group  Tracheata  as  usually 
limited  is  polyphyletic  in  origin. 

1 Kishenouye  (’90)  claims  that  in  the  Araneina  both  the  Malpighian  tubules  and 
the  stercoral  pocket  are  derivatives  of  the  mesoderm,  the  cavity  of  the  latter  being 
the  coelom  of  that  region  of  the  body.  This  is  on  its  face  improbable.  It  would 
seem  that  the  failure  of  many  investigators  to  recognize  that  these  tubules 
are  entodermal  in  origin  was  due  to  the  fact  that  since  they  were  kno\\m  to  be 
ectodermal  in  the  Hexapods,  they  have  been  used  as  regional  tests,  the  fact  that 
they  arose  from  a certain  part  of  the  alimentary  canal  being  sufficient  reason  for 
regarding  that  portion  as  proctodeal.  Beddard’s  view  (’89)  that  the  Malpighian 
tubes  are  derived  from  nephridia  secures  no  support  in  the  Arachnida. 


246 


KINGSLEY, 


[VOL.  VIII. 


Again,  as  I previously  argued,  the  existence  of  similarly 
placed  tubules  in  certain  Amphipods  can  be  advanced  as  an 
argument  for  the  closer  association  of  the  Arachnids  and  the 
Crustacea.  Still  exact  knowledge  of  these  tubules,  in  the  Am- 
phipoda  is  lacking.  Nebeski  (’80)  regards  them  as  diverticula 
of  the  hind-gut,  while  Spencer  (’85),  upon  histological  grounds, 
regards  them  as  outgrowths  of  the  mesenteron  and  hence,  like 
those  of  the  Arachnids,  entodermal.  It  must  be  said,  however, 
that  this  evidence  is  hot  conclusive,  as  the  limits  of  the  hind- 
gut  are  not  clearly  ascertained,  and  the  assumption  that  these 
are  entodermal  is  based  upon  the  absence  of  a chitinous  cuticule 
(Spencer,  ’85,  PI.  XIII,  Fig.  2)  and  by  a break  in  the  character 
of  the  epithelium  in  the  alimentary  canal  at  the  point  of  origin 
of  these  tubes,  the  tubes  themselves  apparently  belonging  to 
the  anterior  portion. 

All,  then,  that  can  be  argued  from  the  various  structures 
known  as  Malpighian  tubules  is  that  homoplastic  and  analogous 
organs,  rather  than  exact  homologues,  are  included  under  this 
name  ; that  their  existence  in  both  Arachnids  and  Hexapods 
is  an  argument  against  the  close  association  of  these  forms 
and  that  their  absence  in  Limulus  can  only  be  used  as  a 
negative  argument  of  little  weight.  In  this  connection  the 
conditions  figured  on  PL  XIII,  Fig.  88,  deserve  more  detailed 
study  in  later  stages. 

The  presence  of  salivary  glands  in  the  ‘‘ Tracheates  ” and 
their  absence  from  the  “ Branchiates  ” (Crustacea,  Limulus) 
is  possibly  to  be  explained  by  the  different  method  of  life  of 
the  members  of  the  two  groups — aquatic  in  the  latter,  ter- 
restrial in  the  former.  It  is,  however,  to  be  noted  that  sali- 
vary glands  have  been  recognized  in  Astacus  (Lang,  ’89,  p. 
344),  while  renewed  studies  must  be  made  of  the  so-called 
salivary  glands  of  the  Arachnida  before  we  are  certain  of 
their  homology  with  those  of  the  Hexapods.  Several  organs 
which  have  been  called  salivary  glands  among  the  spiders  and 
their  allies  have  been  shown  to  be  coxal  glands  {i.e.  nephridia) 
or  poison  glands,  and  it  is  possible  that  all  of  these  organs 
may  have  different  homologies  than  those  indicated  by  the 
name  usually  applied  to  them. 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  247 

There  is  one  point  of  resemblance  between  the  Arachnids 
and  the  Hexapods  which  may  have  no  inconsiderable  weight. 
In  the  Scorpions  as  in  the  Hexapods,  the  embryo  develops 
those  as  yet  unexplained  foetal  membranes  which  so  closely 
simulate  those  of  the  higher  vertebrates.  It  may  be  that 
here,  as  in  other  places,  we  have  similar  but  not  identical 
organs.  The  accounts  of  their  development  in  the  Arachnids 
by  Metschnikoff,  Kowalevsky  and  Schulgin,  and  Laurie  differ 
considerably,  and  until  we  know  something  of  the  ancestry 
and  real  meaning  of  the  structures  which  are  united  under 
this  head  we  cannot  be  certain  of  the  taxonomic  value  to  be 
placed  upon  them.  It  may  be  noted  here  that  the  structures 
described  by  Bruce  (’87)  as  occurring  in  the  spiders  are  in  all 
probability  not  amnion  and  serosa,  but  either  the  invaginations 
in  connection  with  the  brain  or  the  inpushing  to  form,  the 
median  eye. 

The  Classification  of  the  Arthropod  a. 

As  a result  of  my  studies  it  would  seem  as  if  the  Arthropoda 
must  be  divided  in  some  such  manner  as  that  here  given  : — 

Phylum  Arthropoda. 

Sub-Phylum  Branchiata. 

Class  Crustacea. 

Class  Acerata. 

Sub-Class  Gigantostraca. 

Sub-Class  Arachnida.  ^ 

Sub-Phylum  Insecta. 

Class  Hexapoda. 

Class  Chilopoda. 

Sub-Phylum  Diplopoda  (Chilognatha). 

IncertcB  Secies. 

Pauropoda. 

Pycnogonida. 

Trilobitae. 

Tardigrada. 

Malacopoda. 

1 The  attempt  by  Haller  (’81)  to  separate  the  Acarina  ajs  a distinct  class  hardly 
seems  warranted. 


248 


KINGSLEY. 


[VOL.  VIII. 


While  the  present  is  not  the  proper  opportunity  to  support 
the  above  classification  in  detail,  a slight  amount  of  explana- 
tion seems  necessary  with  regard  to  some  of  the  novelties  in- 
troduced above. 

It  has  been  shown,  I think  conclusively,  that  the  relation- 
ship existing  between  the  Arachnida  and  the  Xiphosures  is 
very  close  ; that  they  have  more  affinities  with  each  other 
than,  on  the  one  side,  the  Arachnida  have  with  the  other 
‘‘  Tracheates,”  or  than  Limulus  has,  on  the  other  hand,  to  the 
Crustacea.  For  the  class  formed  by  the  union  of  these  forms 
I proposed,  eight  years  ago,  the  name  Acerata,^  a modification 
of  the  term  Acera  applied  by  Latreille  to  the  Arachnida 
alone.  For  the  sub-class  containing  the  Xiphosures  and  the 
Eurypterina  I have  followed  Dohrn  in  modifying  and  adopting 
the  term  Gigantostraca  of  Haeckel.  For  essentially  the  same 
group  Packard  has  proposed  at  different  times  the  names 
Palaeocarida  and  Podostomata,  while  Steinmann  and  Doderlein 
(Elemente  der  Palaontologie)  have  applied  to  the  same  associa- 
tion of  forms  the  term  Palaeostraca. 

The  resemblances  between  the  Acerata  and  the  Crustacea  are 
much  closer  than  those  between  either  and  any  of  the  other 
groups  of  Arthropods,  and  from  the  fact  that  in  each  respiration 
is  effected  by  gills  or  by  their  homologues,  developed  in  all 
cases  as  membranous  expansions  of  the  limbs,  the  older  term 
Branchiata,  used  with  enlarged  scope,  seems  most  applicable  to 
the  group  or  sub-phylum  formed  by  their  union. 

The  so-called  Myriapoda  seems  to  be  a heterogeneous  asso- 
ciation of  forms,  polyphyletic  in  origin,  and  only  associated 
together  through  the  possession  of  many  locomotor  appendages. 
On  the  other  hand,  the  resemblances  between  the  Chilopods 
.and  the  Hexapods  are  far  more  nurnerous  and  of  far  more  sig- 

1 Since  this  use  of  the  term  Acerata  Lankester  has  employed  it  (’90)  as  equiva' 
lent  to  the  term  Branchiata  as  limited  in  this  article.  Cholodkowsky  (’91)  objects 
to  my  group  Acerata  apparently  more  upon  the  inapplicability  of  the  term  than 
from  any  objection  to  the  association  of  the  Arachnids  with  the  Xiphosures.  As 
I think  I have  strengthened  the  ground  for  such  union  in  the  present  article  a 
name  for  the  group  becomes  necessary,  and  as  in  both  the  Xiphosures  and  the 
Arachnids  functional  (if  not  morphological)  antennae  are  entirely  lacking,  I may 
be  permitted  to  continue  the  use  of  the  term. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


249 


nificance  than  those  between  Chilopods  and  Diplopods  (Chilog- 
naths).  This  was  pointed  out  some  years  ago  by  Mr.  Pocock 
(’87),  of  the  British  Museum,  while  I,  independently  (’88), 
stated  similar  conclusions. 

Until  we  know  more  of  both  the  structure  and  the  ontogeny 
of  the  Myriapod  forms  the  correctness  of  this  view  cannot  be 
regarded  as  settled,  but  in  the  present  state  of  bur  knowledge 
the  following  facts  seem  important : 

The  Diplopod  head  bears,  besides  the  antennae,  but  two  pairs 
of  appendages,  — a pair  of  mandibles  and  a lower  lip,  com- 
posed of  a pair  of  coalesced  maxillae.^  In  the  Chilopod  the 
conditions  are  as  in  the  Hexapod,  two  pairs  of  maxillae  being 
present. 

In  the  Chilopods  as  in  the  Hexapods,  each  somite  bears  a 
single  pair  of  appendages,  while  in  the  Diplopods  the  majority 
of  the  segments  bear  two  pairs  of  appendages,  and  the  re- 
searches of  Heathcote  show  that  each  segment  is  in  reality 
composed  of  two  coalesced  somites,  a condition  without  parallel 
elsewhere  in  the  Arthropoda.  In  the  Chilopods  there  is  a 
wide  sternum  separating  the  coxae  of  the  ambulatory  append- 
ages ; in  the  Diplopods  the  coxae  are  approximate,  and  the 
sternum  is  exceedingly  narrow,  or  even  entirely  absent. 

In  the  Chilopods  the  stigmata,  a pair  to  a somite,  are  lateral 
(dorsal  in  Scutigera),  and  are  placed  above  and  outside  the 
insertion  of  the  limbs,  exactly  as  in  the  Hexapods.  The 
tracheae  which  arise  from  them  are  branched,  and  the  intima 
is  thrown  into  a well  developed  spiral  thickening  as  in  the  six- 
footed insects.  In  the  Diplopoda,  on  the  other  hand,  the  stig- 
mata are  beneath  or  even  in  the  coxae,  while  the  tracheae 
(except  in  the  Glomeridae)  are  tufted  and  unbranched,  and  the 
thickening  of  the  intima  is  poorly  developed. 

In  the  Diplopods  there  are  well  developed  foramina  repug- 
natoria  upon  the  sides  of  each  somite  of  the  body.  Such 

1 The  attempt  made  to  show  that  this  lower  lip  is  a “ gnathochilarium  ” com- 
posed of  the  two  coalesced  lower  jaws,  or  first  and  second  maxillae  of  the  Chilog- 
naths  receives  no  support  from  the  embryology  of  Julus  (Heathcote  ’88),  where 
there  is  but  a single  somite  when  the  hypothesis  calls  for  two.  Further  the  inner- 
vation of  the  sense  organs  of  the  lower  lip  {cf.  vom  Rath.  ’86,  PI.  XX,  Fig.  i) 
shows  that  but  a single  pair  of  appendages  is  concerned  in  the  part. 


250 


KINGSLEY. 


[VOL.  VIII. 


structures  are  absent  from  the  Chilopods  (as  from  the  Hexa- 
pods), except  in  a few  Geophilidae,  where  repugnatorial  glands 
occur,  opening  by  foramina  in  the  mid-ventral  line. 

In  the  Chilopods  the  reproductive  organs  consist  of  paired  ^ 
gonads  situated  above  the  alimentary  canal  and  opening  to  the 
exterior  by  ducts  which  are  at  first  paired,  but  which  later 
unite  into  a common  tube  which  leads  to  a single  external 
opening  situated  in  the  penultimate  segment  of  the  body.  In 
the  Hexapods  the  conditions  are  almost  exactly  the  same  ; the 
gonads  are  dorsal,  the  genital  ducts  unite  (except  in  Ephem- 
eridae),  and  there  is  a single  external  opening,  always  at  the 
posterior  end  of  the  abdomen.  In  both  Hexapods  and  Chilo- 
pods the  spermatozoa  are  motile.  In  the  Diplopods  there  is 
a single  unpaired  gonad  situated  beneath  the  alimentary  canal, 
and  the  genital  duct,  passing  forward,  divides  into  two,  each 
of  which  has  its  own  opening  at  the  bases  of  the  legs  of  the 
second  post  cephalic  segment.  The  spermatozoa  are  quiescent. 

We  know  so  little  of  the  embryology  of  the  Myriapods  that 
the  aid  of  development  can  be  had  to  only  a slight  extent  in 
our  comparisons,  but  the  facts  which  it  affords  seem  important. 
In  the  Chilopods  the  embryo  escapes  from  the  egg  with  nu- 
merous ambulatory  appendages,  a pair  to  each  somite.  The 
same  is  true  of  the  typical  Hexapods,  all  later  observers  agree- 
ing that  a polypod  precedes  a hexapod  condition.  The  young 
Diplopod  escapes  from  the  egg  in  a hexapod  condition,  and 
the  presence  of  these  six  legs  has  been  seized  upon  as  a proof 
of  the  near  association  of  these  forms.  An  exact  comparison, 
however,  seems  to  show  that  the  two  are  in  reality  very  unlike 
as  appears  in  the  following  table.^ 

1 Single  in  Scolopendra. 

2 As  nothing  is  known  of  the  existence  of  a tritocerebral  segment  in  the  Diplo- 
pods, the  comparison  can  only  be  made  upon  the  basis  of  the  appendages  of  the 
adult.  If  the  tritocerebral  segment  should  prove  lacking  in  the  millepeds  the 
contrast  will  prove  stronger  than  it  now  is.  The  statement  of  the  Diplopod 
appendages  is  based  upon  Heathcote  (’88). 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


251 


Hexapod. 

Diplopod. 

Appendage  I 

Antenna 

Antenna  "] 

a 

II 

Mandible 

►Head 

Mandible  ^Head 

u 

III 

Maxilla  i 

Lower  Lip  J 

» 

IV 

Maxilla  2 

Fopt  I 

li 

V 

Thoracic  Foot  i 'I 

Absent 

a 

VI 

Thoracic  Foot  2 ^Thorax 

Foot  2 

ii 

VII 

Thoracic  Foot  3 J 

Foot  3 

u 

VIII 

Abdominal  Foot  i 

Absent 

u 

IX 

Abdominal  Foot  2 

Absent 

The  result  of  these  comparisons  is  sufficient,  I think,  to 
justify  the  dismemberment  of  the  old  group  Myriapoda  and  the 
association  of  the  Chilopoda  with  the  Hexapoda  in  a group  to 
which  the  much  abused  term  Insecta  may  be  applied,  while, 
until  more  definite  knowledge  be  obtained,  the  Diplopoda  must 
be  allowed  to  stand  alone.  The  position  of  the  Pauropoda  is 
as  yet  very  uncertain  as  we  are  almost  entirely  ignorant  of  their 
internal  structure.  In  the  tendency  towards  a fusion  of  somites, 
in  the  lack  of  a second  pair  of  maxillae,  and  in  the  positions  of 
the  external  paired  openings  of  the  genital  ducts  at  the  base  of 
the  second  pair  of  ambulatory  appendages  they  show  undoubted 
affinities  with  the  Diplopoda;  but  the  peculiar  triramous  an- 
tennae and  especially  the  characters  of  the  hexapod  young  (if 
Ryder’s  (79)  figure  of  the  young  of  Eurypauropus  be  correct) 
militate  against  this  view. 

The  Malacopoda^  (Peripatus)  are  also  frequently  placed  in 
close  association  with  the  Myriapods,  but  it  may  be  that 
their  status  as  members  of  the  Arthropod  phylum  is  not 
beyond  question.  In  the  following  points  they  differ  from  all 
Tracheates  ” and  also  from  all  Arthropods,  while  in  just 
these  same  points  they  show  affinities  with  the  Annelids:  — 
The  presence  of  functional  nephridia  in  each  body  segment ; 
the  presence  of  well  developed  coxal  glands  {cf.  setiparous 
glands  of  Annelids) ; the  existence  of  an  outer  circular  muscular 

1 Malacopoda,  Blanchard  1847;  Onychophora,  Grube  1853;  Protracheata,  Moseley 
1874. 


252 


KINGSLEY. 


[VOL.  VIII. 


layer  in  the  body  wall ; the  absence  of  striation  from  all  muscles 
except  those  of  the  mouth  parts;  the  presence  of  cilia  in  the 
alimentary  canal  and  in  the  nephridia;  the  situation  of  the 
antennae  as  outgrowths  from  the  primitively  preoral  region  icf. 
supra  p.  232);  the  muscular  nature  of  the  pharynx,  unlike  that 
of  any  Arthropod  and  strikingly  like  that  of  certain  Chaetopods. 
The  eyes  too  are  unlike  the  visual  organs  of  any  other  Arthropod 
but  as  figured  by  Balfour  they  closely  resemble  these  organs  in 
Autolytus.  It  is  noticeable  that  Balfour  has  described  (’83)  a 
pair  of  problematical  organs  upon  the  lower  surface  of  the 
brain  of  P.  capensis  (the  auditory  organs  of  Grube,  ’53).  Sedg- 
wick has  shown  that  these  organs  are  developed  by  an  invagin- 
ation of  the  cerebral  surface  while  the  slight  account  given  by 
Balfour  of  the  adult  structure  at  once  suggests  a degenerate 
eye  formed  upon  the  same  plan  as  the  functional  one.  In 
Autolytid  worms  a second  pair  of  eyes  occur  at  the  same  point. 

On  the  other  hand  the  Arthropod  structures  are  not  to  be 
ignored;  the  tracheae;  the  appendicular  jaws;  the  setting  aside 
of  a pair  of  nephridia  for  genital  ducts;  the  heart,  with  several 
paired  ostia,  enclosed  in  a pericardium;  the  lacunar  circulation, 
and  the  reduced  coelom. 

To  the  discussion  of  the  position  of  the  Pycnogonids  and 
the  Tardigrades  I can  add  nothing.  Morgan  (’90)  has,  it  seems, 
shown  that  the  Pycnogonids  present  certain  features  both  in 
ontogeny  and  in  adult  structure  which  can  only  be  paralleled  in 
the  Arachnids,  while  the  Tardigrades  may  have  no  other  claim 
upon  a position  in  the  same  group  than  that  afforded  by  their 
eight  ambulatory  feet. 

For  many  years  the  general  consensus  of  opinion  has  been 
to  the  effect  that  the  Trilobites  are  closely  related  to  the 
Xiphosures.  We  unfortunately  know  but  little  regarding  the 
structure  of  the  Trilobites  aside  from  the  features  presented  by 
the  dorsal  surface.  For  our  knowledge  of  the  appendages  we 
have  to  thank  the  papers  of  Billings  (’70)  and  Walcott  (’81  and 
’84).  From  Billings’  paper  (and  from  electrotypes  of  his 
specimens  which  I have  studied)  we  can  learn  but  little  except 
the  presence  of  jointed  appendages.  Walcott’s  researches 
tell  much  more,  but  the  facts  which  they  have  added  all  are 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


253 


opposed  to  the  close  association  of  the  Trilobites  with  Limulus. 
The  body  of  Limulus,  it  must  be  remembered,  possesses  an 
anterior  cephalothorax  bearing  six  pairs  of  circumoral  chelate 
appendages  without  differentiation  into  exopodite  and  endo- 
podite,^  and  with  no  trace  of  gills.  In  the  Trilobite  but  four 
pairs  of  appendages  occur  in  this  region.  Hence,  if  the 
^‘head”  of  the  Trilobite  is  to  be  compared  with  the  cephalo- 
thorax of  Limulus  we  must  assume  — for  which  we  have  as  yet 
no  evidence  — that  two  pairs  of  appendages  have  been  lost 
from  the  Trilobite.  The  abdomen  of  Limulus  bears  six  pairs  of 
broad  leaf-like  appendages,  the  posterior  five  pairs  having 
lamillate  gill  books  upon  the  posterior  surface.  In  the  corre- 
sponding region  of  the  Trilobite,  the  thorax,  we  have  an  indefi- 
nite number  of  somites,  each  of  which  bears  the  typical 
CriLstacean  foot,  consisting  of  basiopodite,  exopodite,  and  en- 
dopodite,  and,  outside  the  exopodite,  occupying  the  same  posi- 
tion as  the  gill  in  the  Decapod,  a straight  or  curiously  coiled 
structure  interpreted  by  Walcott  as  the  gill.  In  the  horse- 
shoe crab  the  abdominal  region  extends  to  the  anus,  behind 
which  comes  the  non-segmental  tail.  In  the  Trilobites  the 
thorax  is  followed  by  a segmented  pygidium  on  which  the 
series  of  appendages^  is  continued  to  the  end,  and  there  is  no 
evidence  of  a supra-anal  telson. 

The  necessary  conclusion  is  that  the  appendages  of  the  Trilo- 
bite vary  in  7Uimber  and  differ  totally  in  structure  from  those  of 
Limulus,  and  the  association  of  the  Trilobites  with  the  Xipho- 
sura  is  not  warranted  by  our  present  state  of  knowledge.  The 
trilobites  would  appear  to  be  true  Crustacea,  the  sessile  eyes 
and  general  shape  of  the  body  allying  them  to  the  Isopods, 

^ The  flabellum  of  the  sixth  appendage  cannot  be  considered  as  a representative 
of  the  expedite  since  it  develops  later  than  the  rest  of  the  limb,  develops  inde- 
pendently of  it  and  only  in  the  later  embryonic  stages  does  the  base  of  the  leg 
enlarge  so  that  it  is  included. 

2 Walcott  continues  the  series  of  ambulatory  appendages  through  thi«  region, 
but  Professor  Mickleborough  (’83),  who  found  the  specimen  forming  the  basis 
of  Walcott’s  second  paper  (’84),  thinks  that  these  pygidial  appendages  were 
lamellar.  Henry  Woodward  (’70)  describes  what  he  considers  as  the  jointed 
palpus  of  one  of  the  “ maxillae  ” of  Asaphus  with  seven  articulations  beyond  the 
basal  joint. 


254 


KINGSLEY. 


[VOL.  VIII. 


while  the  biramous  appendage  and  the  epipodial  gills  would 
rather  indicate  relationships  between  the  Phyllopods  and  the 
lower  Podophthalmia.  It  will  prove  a profitable  field  for  some 
student  of  Arthropod  morphology  to  repeat  Walcott’s  earlier 
investigations.  It  will  be  noticed  that  I place  the  grounds  for 
rejection  of  the  association  of  Limulus  and  the  Trilobites  upon 
different  grounds  from  those  advanced  by  Owen  (’72,  pp.  491- 
493)  and  the  older  Milne  Edwards  (’8l). 

Tufts  College,  Mass., 

Mar.  12,  1893. 


No.  2.] 


THE  EMBRYOLOGY  OF  LI M ULUS. 


255 


BIBLIOGRAPHY. 

’85  Ayers,  H.  On  the  carapax  and  sternum  of  Decapod  Crustacea.  Bull. 
Essex  Inst.,  x.v\\.  1885. 

’80  Balfour,  F.  M.  Notes  on  the  development  of  the  Araneina.  Quart. 
Jour.  Micr.  Sci.,  xx.  1880. 

’83  Balfour,  F.  M.  The  anatomy  and  development  of  Peripatus  capen- 
sis.  Quart.  Jour.  Micr.  Sci.,  xxA\\.  1883. 

’71  Van  Beneden,  Edouard.  [No  title.]  Comptes  Rendus  Soc.  Ent. 
Belg.,  p.  ix.  1871. 

’89  Beddard,  F.  E.  On  the  possible  origin  of  the  Malpighian  tubules 
in  the  Arthropods,  Annals  and  Mag.  Nat.  Hist.,  iv.  1889. 

’83  Benham,  W.  B.  S.  Description  of  the  muscular  and  endoskeletal 
systems  of  Limulus.  Trans.  Lmn.  Socy.,  xi.  1883. 

’92  Bernard,  H,  M.  An  endeavor  to  show  that  the  trachem  of  the 
Arthropoda  arose  from  setiparous  sacs.  Zoolog.  Jahrbiicher,  Abth. 
f.  Anat.,  V.  1892. 

’93  Bernard,  H.  M.  Additional  notes  on  the  origin  of  tracheae  from 
setiparous  glands.  Annals  a7id  Mag.  Nat.  Hist.,  xi.  1893. 

’90  Berteaux,  L.  Le  poumon  des  Arachnides.  La  Cellule,  v.  1890. 

’70  Billings,  E.  Notes  on  some  specimens  of  lower  Silurian  Trilobites. 
Quar.  Jour.  Geol.  Socy.,  xxvi.  1870. 

’92  Birula,  a.  Beitrage  zur  Kenntniss  des  anatomischen  Baues  der 
Geschlechtsorgane  bei  den  Galeodiden.  Biol.  Centralbl.,  xii.  1892. 

’85  Brooks,  W.  K.  and  Bruce,  A.  T.  Abstract  of  researches  on  the 
embryology  of  Limulus  polyphemus,  Johns  Hopkins  Circ.,  v.  1885. 

’87  Bruce,  A.  T.  Observations  on  the  embryology  of  Insects  and  Arach- 
nids. Baltimore,  1887. 

’91  Bumpus,  H,  C,  The  embryology  of  the  American  lobster.  Jour, 
of  Morph.,  V.  1891. 

'90  Carriere,  J.  Die  Entwicklung  der  Mauerbiene  (Chalicodoma  muraria). 
Arch.  f.  mikr.  Anat.,  xxxv.  1890. 

’75  Chun,  C.  Bau,  Entwicklung  und  physiologische  Bedeutung  der  Rectal- 
driisen  bei  den  Insekten.  Abh.  Sc7ikcnb.  Gcsell.  Fra7ikfurt,  x.  1875. 

’91  Cholodkowsky,  N.  Die  Embryonalentwicklung  von  Phyllodromia 
(Blatta)  germanica.  Mc7n.  Acad.  St.  P etersburg,  VII,  xxxviii.  1891. 

’87  Claus,  C.  Prof.  E.  Ray  Lankester’s  Artikel  “ Limulus  an  Arachnid  ” 
und  die  auf  denselben  gegriindeten  Pratensionen  und  Anschuldigung- 
en.  Arb.  zool.  Inst.  Wic7i,  vii.  1887. 

’92  Claus,  C.  Beitrage  zur  Kenntniss  der  siisswasser  Ostracoden.  Arb. 
zool.  Inst.  Wien,  x.  1892. 

’80  Croneberg,  a.  Ueber  die  Mundtheile  der  Arachniden.  Arch.  f. 
Naturgcsch.,  xlvi.  1880. 


256  KINGSLE  V.  [ VoL.  V 1 1 1 . 

’52  Dana,  J.  D.  Crustacea  of  the  U.  S.  Exploring  Expedition,  Part  I. 
Philadelphia,  1852. 

’87  Eisig,  H.  Die  Capitelliden  des  Golfes  von  Neapel.  Fauna  u.  FI. 
Golfes  Neapel^  y;!\v.  1887. 

’88  Faussek,  V.  Entwicklung  der  Geschlechtsorgane  bei  Phalangium. 
Biol.  Cblt.,  viii.  1888. 

’91  Faussek,  V.  Zur  Embryologie  von  Phalangium.  ZooL  Anz.,  xiv. 

1891. 

’79  Grobben,  C.  Die  Entwicklungsgeschichte  der  Moina.  Arb.  zool. 
Fist.,  Wien,  ii.  1879. 

’53  Grube,  E.  Ueber  den  Bau  von  Peripatus  edwardsii.  Millie  As 
Archiv.  1853. 

’85  Gulland,  G.  L.  Evidence  in  favor  of  the  view  that  the  coxal 
gland  of  Limulus  and  other  Arachnids  is  a modified  nephridium. 
Quar.  Jour.  Micr.  Sci.,  xxv.  1885. 

’81  Haller,  G.  Die  Mundtheile  und  systematische  Stellung  der  Milben. 
Zool.  Anz.,  iv.  1881. 

’88  Heathcote,  F.  G.  The  postembryonic  development  of  Julus  ter- 
restris.  Phil.  Trans.,  179B.  1888. 

’89  Heider,  K.  Die  Embryonalentwicklung  von  Hydrophilus.  Jena. 
1889. 

’82  Henking,  H.  Beitrage  zur  Anatomie,  Entwicklungsgeschichte  und 
Biologie  von  Trombidium.  Zeitschr.  f.  wiss.  zool.,  xxxvii.  1882. 
’92  Herrick,  F.  H.  Alpheus:  a study  in  the  development  of  Crustacea. 

Memoirs  National  Acad.  Sci.,  v.  1892. 

’90  Heymons,  R.  Ueber  die  hermaphroditische  Anlage  der  Sexualdriisen 
beim  Mannchen  von  Phyllodromia  (Blatta).  Zool.  Anz.,  xiii.  1890. 
’87  Houssay,  F.  Sur  la  lacune  sanguine  perinerveuse  chez  les  scorpions. 
C.  R.,  civ.  1887. 

’91  Jaworowski,  a.  Ueber  die  Extremitaten  bei  den  Embryonen  der 
Arachniden  und  Insecten.  Zool.  Anz.,  xiv.  1891. 

’92  Jaworowsky,  a.  Ueber  die  Extremitaten,  deren  Driisen  und  Kopf- 
segmentierung  bei  Trochesa  singoriensis.  Zool.  Anz.,  xv.  1892. 
’84  VON  Kennel,  J.  Entwicklungsgeschichte  von  Peripatus.  Arb.  zooL 
zoot.  Inst.,  Wurzburg,  vii.  1884. 

’85  Kingsley,  J.  S.  Notes  on  the  embryology  of  Limulus.  Quar.  Jour, 
Micros.  Sci.,  xxv.  1885. 

’88  Kingsley,  J.  S.  The  classification  of  the  Myriapods.  Am.  Natur- 
alist, xxii.  1888. 

’89  Kingsley,  J.  S.  The  development  of  Crangon  vulgaris.  III.  Bull. 
Essex.  Inst.,  xxi.  1889. 

’90  Kingsley,  J.  S.  The  ontogeny  of  Limulus.  Am.  Naturalist,  xxiv 
and  Zool.  Anz.,  xiii.  1890. 

’92  Kingsley,  J.  S.  The  embryology  of  Limulus.  Jour.  Morphol.,  vii. 

1892. 


No.  2.]  THE  EMBRYOLOGY  OF  LIMULUS.  257 

’90  Kishinouye,  K.  On  the  development  of  the  Araneina.  Jour.  Coll. 
Science^  Univ.  of  fajan.,  iv.  1890. 

’91  Kishinouye,  K.  On  the  development  of  Limulus  longispina.  Jour. 
Coll.  Sci.,  Univ.  Japan.,  v.  1891. 

’86  Kowalevsky,  A.  and  Schulgin,  M.  Zur  Entwicklungsgeschichte 
des  Skorpions.  Biol.  Cdll.,  vi.  1886. 

’92  Korschelt,  E.  and  Heider,  K.  Lehrbuch  der  vergleichenden  Entwick- 
lungsgeschichte der  wirbellosen  Thiere.  Zweites  Heft.  Jena.  1892. 
’89  Lang,  A.  Lehrbuch  der  vergleichenden  Anatomie.  Zweite  Abth. 
Jena,  1889. 

’81  Lankester,  E.  R.  Limulus  an  Arachnid.  Q.J.M.S..,xx[.  1881. 

’82  Lankester,  E.  R.  On  the  coxal  glands  of  Scorpio  hitherto  undescribed 
and  corresponding  to  the  brick-red  glands  of  Limulus.  Broc.  Royal 
Socy.^  xxiv.  1882. 

’84  Lankester,  E.  R.  On  the  skeleto-trophic  tissues  and  coxal  glands 
of  Limulus,  Scorpio  and  Mygale.  Q.J.  M.  S’.,  xxiv.  1884. 

’85  Lankester,  E.  R.  A new  hypothesis  as  to  the  relationship  of  the  lung- 
book  of  Scorpio  to  the  gill-book  of  Limulus.  Q-J.  M.  S.,  xxv.  1885. 
’90  Lankester,  E.  R.  Article  “Zoology.”  Encyc.  Brit,  9th  edit,  Vol. 
xxiv. 

’90  Laurie,  M.  The  embryology  of  a scorpion.  Q.  J.  M.  S.,  xxxi.  1890. 
’91  Laurie,  M.  Some  points  in  the  development  of  Scorpio  fulvipes. 
Q.J.  M.  S.,  xxxii.  1891. 

’92  Laurie,  M.  On  the  development  of  the  lung-books  of  Scorpio  ful- 
vipes. Zool.  A71Z.,  XV.  1892. 

’49  Leuckart,  R.  Ueber  den  Bau  und  die  Bedeutung  der  sog.  Lungen 
bei  den  Arachniden.  Zeitschr.  f.  wiss.  Zool.  1849. 

’89  Leydig,  F.  Ueber  Argulus  foliaceus.  Arch.  f.  mikr.  Anal..,  xxxiii. 
1889. 

’92  Lebedinsky,  J.  Die  Entwicklung  der  Coxaldriisen  bei  Phalangium. 
Zool.  Anz.,  XV.  1892. 

’86  Locy,  W.  a.  Observations  on  the  development  of  Agelena  naevia. 
Bull.  Mus.  Comp.  Zool..,  xii.  1886. 

’87  Loman,  J.  C.  C.  Altes  und  Neues  iiber  das  Nephridium  (die  Coxal- 
driise)  der  Arachniden.  Bijdr.  tot  Dierkunde  N.  A.  M.  14.  afl. 
’86-’87  Loman,  J.  C.  C.  Ueber  die  morphologische  Bedeutung  der  sog. 
malpighi’schen  Gefasse  der  echten  Spinnen.  Tijdsch.  nederl.  Die^'k. 
Vereen.,  II,  i.  1886-1887. 

’84  MacLeod,  J.  Recherches  sur  la  structure  et  la  signification  de  I’appareil 
respiratoire  des  Arachnides.  Arch.  Biol..,  v.  1884. 

’84a  MacLeod,  J.  Sur  I’existence  d’une  glande  coxale  chez  les  Phalan- 
gides.  Bull.  Acad.  Belg..,  Ill,  viii.  1884. 

’92  Marched,  Paul.  The  coxal  gland  of  the  scorpion  and  its  morpholog- 
ical relations  with  the  excretory  organs  of  the  Crustacea.  A7in.  a7id 
Mag.  Nat.  Hist..,  x.  1892  from  C.R.,  cxv.  1892. 


258 


KINGSLEY. 


[VOL.  VIII. 


’71  Metschnikoff,  E.  Embryologie  des  Scorpions.  Zeitschr.  f.  wiss. 
Zool..,  xxi.  1871. 

’83  Mickleborough,  J.  Locomotive  appendages  of  Trilobites.  Am.A^al, 
xvii,  1883.  From  Jour.  Cincinnati  N.  H.  Socy. 

’72  Milne-Edwards,  H.  Recherches  sur  I’anatomie  des  Limulus.  Ann. 

Sci.  Nat..,  V,  xvii.  1872.  Also  Miss.  Sci.  Mex.  1873. 

’81  Milne-Edwards,  H.  Compte  rendu  des  nouvelles  recherches  de 
M.  Walcott  relatives  a la  structure  des  Trilobites.  Ann.  Sci.  Nat., 
VI,  xii.  1881. 

’91  Morgan,  T.  H.  A Contribution  to  the  embryology  and  phylogeny  of 
the  Pycnogonids.  Studies  Biol.  Lau.  Johns  Hojkins  U7tiv.,  v. 
1891. 

’87  Morin,  I.  Zur  Entwicklungsgeschichte  der  Spinnen.  Biol.  Cblt.,  vi. 
1887. 

’74  Moseley,  H.  N.  On  the  structure  and  development  of  Peripatus 
capensis.  Phil.  Trans.,  clxiv.  1874. 

’89  Narayanan,  M.  Notes  on  the  anatomy  of  scorpions.  Q-J>  M.  S., 

XXX.  1889. 

’80  Nebeski,  O.  Beitrage  zur  Kenntniss  der  Amphipoden  der  Adria. 
Arb.  Zool.  List.  Wien,  iii.  1880. 

’43  Newport,  G.  The  nervous  and  circulatory  systems  in  Myriapoda 
and  in  macrurous  Arachnida.  Phil.  Trans.  1843. 

’55  Owen,  R.  Lectures  on  the  Invertebrata.  Second  edition.  London. 
1855. 

’72  Owen,  R.  On  the  anatomy  of  the  American  king-crab.  Trans. 
Lum.  Socy.,  xxviii.  1872. 

’75  Packard,  A.  S.  As  an  undescribed  organ  in  Limulus,  supposed  to 
be  renal  in  its  nature.  Am.  Nat.,  ix.  1873. 

’80  Packard,  A.  S.  The  anatomy,  histology,  and  embryology  of  Limu- 
lus polyphemus.  Anniv.  Mem.  Bost.  Socy.  N.  H.  1880. 

’84  Patten,  W.  The  development  of  Phryganids.  Q.  J.  M.  S.,nM\v. 

’88  Patten,  W.  Studies  on  the  eyes  of  Arthropods.  II.  The  eyes  of 
Acilius.  Jour.  Morjh.,  ii.  1888. 

’89  Patten,  W.  Segmental  sense-organs  of  Arthropods.  Jour.  Morjh., 
ii.  1889. 

’90  Patten,  W.  On  the  origin  of  Vertebrates  from  Arachnids.  Q.  J. 
M.  S.,  xxxi.  1890. 

’85  Pelseneer,  P.  Observations  on  the  nervous  system  of  Apus.  Q.  J. 
M.  S.,  XXV.  1885. 

’86  Plateau,  F.  De  I’absence  de  mouvements  respiratoire  perceptibles 
chez  les  Arachnides.  Arch.  Biol.,  vii.  1886. 

’87  PococK,  R.  I.  On  the  classification  of  the  Diplopoda.  Ann.  and 
Mag.  N.  H.  1887. 

86  VOM  Rath,  O.  Beitrage  zur  Kenntniss  der  Chilognathen.  Inaug. 
Diss.  Bonn.  1 886. 


259 


No.  2.]  THE  EMBRYOLOGY  OE  LIMULUS. 

’86a  voM  Rath,  O.  Die  Sinnesorgane  der  Antennae  und  der  Unterlippe 
der  Chilognathen.  Arch.  f.  Mikr.  Anat..,  xxvii.  i886. 

’77  Reichenbach,  H.  Die  Embryonalanlage  und  erste  Entwicklung  des 
Flusskrebses.  Zeit.  wzss.  Zool.,  xxix.  1877. 

’86  Reichenbach,  H.  Studien  zur  Entwicklungsgeschichte  des  Fluss- 
krebses. Abk.  Senckenb.  Gesell..,  xiv.  1886. 

’79  Ryder,  J.  A.  An  account  of  a new  genus  of  minute  Pauropod 
Myriapods.  Am.  Nat..,  xiii.  1879. 

’87  ScHiMKEWiTSCH,  W.  Etude  sur  le  developpement  des  Araigndes. 
Arch,  de  Biol..,  vi.  1887. 

’88  Sedgwick,  A.  A monograph  of  the  development  of  Peripatus  ca- 
pensis.  Studies  Morph.  Lab.  Cambridge.,  iv.  1888.  From  Q.  J. 
M.  A.,  1885-88. 

’53  SiEBOLD,  Cj  Th.  von.  Anatomy  of  the  Invertebrata,  translated  by 
W.  J.  Burnett.  Boston.  1853. 

’92  Sinclair,  F.  G.  A new  mode  of  respiration  in  the  Myriapods.  Phil, 
Trans.  183  B.  1892. 

’85  Spencer,  W.  B.  The  urinary  organs  of  Amphipoda.  Q.  J.  M.  A., 
XXV.  1885. 

’91  Sturanay,  R.  Die  Coxaldriisen  der  Arachnoiden.  Arb.  Zool.  Lnst. 
Wien.,  ix.  1891. 

’79  Tichomiroff,  a.  Ueber  die  Entwicklungsgeschichte  des  Seiden- 
wurms.  Zool.  Anz.,  ii.  1879. 

’81  Walcott,  C.  D.  The  Trilobite  : new  and  old  evidence  as  to  its 
organization.  Bull.  Mus.  Comp.  Zool.,  vii.  1881. 

’84  Walcott,  C.  D.  Appendages  of  the  Trilobite.  Science.,  hi.  1884. 

’63  Weismann,  a.  Die  Entwickelung  der  Dipteren  im  Ei.  Zeitsch.  f. 
wiss.  Zool.,  xiii.  1863. 

’89  Weldon,  W.  F.  R.  The  coelom  and  nephridia  of  Palasmon  serra- 

tus.  Jour.  Marine  Biol.  Assoc.,  Ab.  S.,  i.  1889. 

91  Weldon,  W.  F.  R.  The  renal  organs  of  certain  Decapod  Crus- 

tacea. Q.  J.  M.  S.,  xxxii.  1891. 

’92  Weldon,  W.  F.  R.  The  formation  of  the  germ  layers  in  Crangon 
vulgaris.  Q- J-  M.  S.,  xxxiii.  1892. 

’89  Wheeler,  W.  M.  The  embryology  of  Blatta  germanica  and  Dory- 
phora  decemlineata.  Jour.  Morph.,  hi.  1889. 

’78  Whitman,  C.  O.  The  embryology  of  Clepsine.  Q.  J,  M.  S.,  xvih. 
1878. 

’79  Wood-Mason,  J.  Morphological  notes  bearing  on  the  origin  of 
Insects.  Trans.  Ezit.  Socy.  Londozi.  1879. 

’70  Woodward,  H.  Notes  on  the  palpus  and  other  appendages  of 

Asaphus.  Quar.  Jour.  Geol.  Socy.,  xxvi.  1870.  See  also  Geol. 

Mag.,  Feby.  1884. 

’41  Zaddach,  E.  G.  De  Apodis  cancriformis,  Bonn.  1841. 


26o 


KINGSLEY, 


EXPLANATION  OF  THE  FIGURES. 
Reference  Letters. 


a. 

Anus. 

an. 

Neural  artery. 

ar. 

Artery. 

av. 

Ventral  (circumneural)  ar- 

tery. 

bs. 

Blood  sinus. 

c.  \-c.  9. 

C celomic  cavities. 

ce. 

Cerebrum. 

ct. 

Cuticula. 

ec. 

Ectoderm  (in  figures  50-52 

edge  of  carapax). 

en. 

Entoderm. 

ent. 

Entapophysis. 

es. 

Entosternite. 

ev. 

Excretory  vescicle  of  nephri- 

dium. 

g: 

Ganglion. 

gl- 

Gill-leaves. 

h or  ht. 

Heart. 

hep. 

Hepatic  duct. 

i. 

Intima  of  heart. 

iv. 

Invaginations  of  nuclei  in 

neural  anlage  {cf.  Fig.  29). 

1. 

Liver  and  liver  lobes. 

lac. 

Lacunae. 

m or  me. 

Mesoderm. 

mes. 

Mesenteron  and  in  later 

stages,  intestine. 

ml. 

Middle  line  of  section. 

mg. 

Marginal  groove. 

mo. 

Mouth. 

ml. 

Metastoma. 

mu. 

Muscles. 

n.'-n.^ 

Segmental  nerves. 

nd. 

Nephridial  duct. 

ne. 

Nephridium. 

n.  a. 

Neural  anlage  (in  Fig.  54, 
the  anterior  loop  of  the 
nephridium). 

no. 

External  opening  of  nephri- 
dium. 

nst. 

Nephrostome. 

oc. 

Ocellus. 

ce. 

CEsophagus. 

cec. 

(Esophageal  commissure. 

on. 

Nerve  to  ocellus. 

op. 

Operculum. 

pd. 

Proctodaeum. 

pe. 

Pavement  epithelium  of  coe- 
lom 5. 

pr. 

Proventriculus. 

ps. 

Pericardial  sinus. 

pst. 

Primitive  streak. 

^g- 

Sympathetic  ganglion. 

sn. 

Sympathetic  nerve. 

sp. 

Splanchnoplure. 

ss. 

Segmental  sense  (?)  organs. 

si. 

Stomodaeum. 

so. 

Somatoplure. 

t. 

Telson. 

X' 

Y olk-entoderm. 

ys. 

Yolk  spherules. 

yx. 

Yolk  cells. 

I-X. 

Somites  and  appendages. 

262 


KINGSLEY. 


EXPLANATION  OF  PLATE  X. 

Fig.  40.  Longitudinal  section  of  an  embryo  with  cephalic  and  caudal  areas 
and  one  intermediate  somite  developed.  The  arrows  mark  the  limits  of  the 
somite. 

Fig.  41.  Through  primitive  streak  in  advance  of  mouth,  Stage  C,  early. 

Fig.  42.  Transverse  through  the  posterior  primitive  streak,  Stage  C,  early. 

Fig.  43.  Transverse  through  the  anterior  end  of  the  primitive  streak 
(“mouth”),  showing  coelom  of  somite  I,  Stage  C,  early. 

Fig.  44.  Through  primitive  streak  and  marginal  groove.  Stage  C,  early. 

Fig.  45.  Longitudinal  section  through  an  embryo  of  Stage  C,  late. 

Fig.  46.  Longitudinal  section  (a  little  oblique)  through  an  embryo,  with  eight 
somites  developed,  showing  a coelomic  cavity  developed  in  each  of  the  anterior 
seven  somites. 

Fig.  47.  Obliquely  transverse  section.  Stage  C,  showing  the  neural  anlage, 
coelom  I,  and  the  segmental  structure  (?  gland)  of  somite  II. 

Fig.  48.  Through  somite  V,  Stage  C,  late.  (Through  a misunderstanding  on 
the*  part  of  the  lithographer  this  and  Figs.  49  and  52,  representing  only  one  side 
of  the  body,  are  so  turned  upon  the  plate  that  the  median  plane  is  oblique.) 

Fig.  49.  Transverse  through  the  abdomen.  Stage  C,  late. 

Fig.  50.  Transverse  through  somite  IV,  showing  the  segmental  structure 
(dorsal  organ)  of  that  somite,  Stage  D. 

Figs.  51,  52,  53.  Transverse  sections  through  somites  V,  VI,  and  VII,  Stage 
E,  showing  the  coelomic  cavities  in  each  and  the  segmental  structures  (?  glands) 
in  V and  VI. 

Fig.  54  a-i.  Modifications  of  the  coelom  of  somite  V,  Stage  H,  into  the  ne- 
- phridium  ; Fig.  54  i is  the  most  anterior.  In  this  n.a.  refers  to  the  anterior  bend 
of  the  nephridial  tube. 

Fig.  55.  Reconstruction  (by  plotting)  of  the  nephridium  of  Fig.  54. 


262 


KINGSLEY, 


EXPLANATION  OF  PLATE  X. 

Eig.  40.  Longitudinal  section  of  an  embryo  with  cephalic  and  caudal  areas 
and  one  intermediate  somite  developed.  The  arrows  mark  the  limits  of  the 
somite. 

Fig.  41.  Through  primitive  streak  in  advance  of  mouth,  Stage  C,  early. 

Fig.  42.  Transverse  through  the  posterior  primitive  streak,  Stage  C,  early. 

Fig.  43.  Transverse  through  the  anterior  end  of  the  primitive  streak 
(“mouth”),  showing  coelom  of  somite  I,  Stage  C,  early. 

Fig.  44.  Through  primitive  streak  and  marginal  groove.  Stage  C,  early. 

Fig.  45.  Longitudinal  section  through  an  embryo  of  Stage  C,  late. 

Fig.  46.  Longitudinal  section  (a  little  oblique)  through  an  embryo,  with  eight 
somites  developed,  showing  a coelomic  cavity  developed  in  each  of  the  anterior 
seven  somites. 

Fig.  47.  Obliquely  transverse  section.  Stage  C,  showing  the  neural  anlage, 
coelom  I,  and  the  segmental  structure  (.?  gland)  of  somite  II. 

Fig.  48.  Through  somite  V,  Stage  C,  late.  (Through  a misunderstanding  on 
the'  part  of  the  lithographer  this  and  Figs.  49  and  52,  representing  only  one  side 
of  the  body,  are  so  turned  upon  the  plate  that  the  median  plane  is  oblique.) 

Fig.  49.  Transverse  through  the  abdomen.  Stage  C,  late. 

Fig.  50.  Transverse  through  somite  IV,  showing  the  segmental  structure 
(dorsal  organ)  of  that  somite.  Stage  D. 

Figs.  51,  52,  53.  Transverse  sections  through  somites  V,  VI,  and  VII,  Stage 
F,  showing  the  coelomic  cavities  in  each  and  the  segmental  structures  (.?  glands) 
in  V and  VI. 

Fig.  54  a-i.  Modifications  of  the  coelom  of  somite  V,  Stage  H,  into  the  ne- 
- phridium  ; Fig.  54  i is  the  most  anterior.  In  this  n.  a.  refers  to  the  anterior  bend 
of  the  nephridial  tube. 

Fig,  55.  Reconstruction  (by  plotting)  of  the  nephridiura  of  Fig.  54. 


o'i4 

ml 


, lournal  of  Morpholofjjj  I hi.  I ’III. 


J.S.KDel. 


mu 


1 ■??  .Af“' 


> ■' 


264 


KINGSLEY, 


EXPLANATION  OF  PLATE  XI. 

Figs.  56,  57,  58,  59  transverse  sections  of  the  nephridium,  about  Stage  H. 
Fig.  56,  is  most  anterior. 

Fig.  60.  Longitudinal  section  of  the  nephridium  after  the  duct  is  open  to  the 
exterior.  The  external  opening  is  plainly  on  the  posterior  surface'  of  the  basal 
joint  of  the  fifth  appendage  while  the  loop  of  the  duct  extends  nearly  to 
somite  III. 

Fig.  61  a-e.  Horizontal  sections  through  the  nephridium,  Stage  I. 

Fig.  61  f and  g.  Reconstructions  (in  wax)  of  the  nephridium  represented  in 
Figs  61  a-e. 

Fig.  62.  Reconstruction  (in  wax)  of  the  nephridium  of  Stage  L.  To  be  com- 
pared with  Gulland’s  (’85)  Fig.  2.  (Is  exaggerated  in  transverse  diameter.) 

Fig.  63.  Longitudinal  section  of  a portion  of  the  abdomen  showing  the  early 
appearance,  by  splitting,  of  somatoplure  and  splanchnoplure  in  that  region.  In 
the  neural  anlage  {n.  a.)  can  be  seen  the  inpushing  of  nuclei  for  rapid  cell  prolifera- 
tion producing  the  pitted  appearance  shown  in  Fig.  29. 

Figs.  64  and  65.  Transverse  sections  of  the  heart  Stage  H,  Fig.  65,  being  the 
more  anterior  and  the  plane  of  section  64  passing  through  appendage  V. 

Fig.  66.  Heart,  transverse.  Stage  I. 

Fig.  67.  Transverse  section  through  abdomen,  the  section  passing  through  the 
operculum,  Stage  I. 

Fig.  68.  From  the  same  series  as  Fig.  67,  but  more  posterior. 

Fig.  69.  Transverse  through  oesophagus,  oesophageal  commissures  and  sternal 
artery,  Stage  I. 


264 


KINGSLEY, 


EXPLANATION  OF  PLATE  XI. 

Figs.  56,  57,  58,  59  transverse  sections  of  the  nephridium,  about  Stage  H. 
Fig.  56,  is  most  anterior. 

Fig.  60.  Longitudinal  section  of  the  nephridium  after  the  duct  is  open  to  the 
exterior.  The  external  opening  is  plainly  on  the  posterior  surface'  of  the  basal 
joint  of  the  fifth  appendage  while  the  loop  of  the  duct  extends  nearly  to 
somite  III. 

Fig.  61  a-e.  Horizontal  sections  through  the  nephridium,  Stage  I. 

Fig.  61  f and  g.  Reconstructions  (in  wax)  of  the  nephridium  represented  in 
Figs  61  a-e. 

Fig.  62.  Reconstruction  (in  wax)  of  the  nephridium  of  Stage  L.  To  be  com- 
pared with  Gulland’s  (’85)  Fig.  2.  (Is  exaggerated  in  transverse  diameter.) 

Fig.  63.  Longitudinal  section  of  a portion  of  the  abdomen  showing  the  early 
appearance,  by  splitting,  of  somatoplure  and  splanchnoplure  in  that  region.  In 
the  neural  anlage  {n.  a.)  can  be  seen  the  inpushing  of  nuclei  for  rapid  cell  prolifera- 
tion producing  the  pitted  appearance  shown  in  Fig.  29. 

Figs.  64  and  65.  Transverse  sections  of  the  heart  Stage  H,  Fig.  65,  being  the 
more  anterior  and  the  plane  of  section  64  passing  through  appendage  V. 

Fig.  66.  Heart,  transverse.  Stage  I. 

Fig.  67.  Transverse  section  through  abdomen,  the  section  passing  through  the 
operculum,  Stage  I. 

Fig.  68.  From  the  same  series  as  Fig.  67,  but  more  posterior. 

Fig.  69.  Transverse  through  oesophagus,  oesophageal  commissures  and  sternal 
artery.  Stage  I. 


Jounuil  of  Morphology  \oLVIL 


J.Si 


266 


KINGSLEY. 


EXPLANATION  OF  PLATE  XII. 

Fig.  70.  Section  posterior  to  that  shown  in  Fig.  69.  The  sternal  arteries  have 
reached  the  nervous  system  and  lie  upon  it.  Beneath  may  be  seen  the  anterior 
extension  o£  the  ventral  part  of  the  neural  artery. 

Fig.  71.  More  posterior,  showing  that  the  neural  artery  is  paired  above  and 
below  and  also  showing  connection  of  dorsal  and  ventral  parts  of  neural  artery. 

Fig.  72.  Horizontal  section,  Stage  L,  through  proventriculus,  sternal  arteries 
and  anterior  end  of  mesenteron  with  ducts  of  the  hepato-pancreas. 

Fig.  73.  From  the  same  series  as  73,  but  passing  through  heart  and  showing 
the  bifurcation  for  sternal  arteries,  an. 

Fig.  74.  Section  from  same  series  as  Figs.  69-71,  passing  through  fifth  pair 
of  legs. 

Fig.  75.  Through  the  abdomen.  Stage  L. 

Fig.  76.  From  the  same  series  as  Fig.  74,  showing  the  connection  of  sternal 
arteries  with  the  heart  and  then  splitting  for  the  stomodaeum. 

Fig.  77.  Reconstruction  (wax)  of  the  anterior  end  of  the  heart,  sternal  arteries 
and  neural  artery.  Stage  K.  At  the  left  is  shown  the  cavity  for  the  ventral  cord  ; 
the  dark  spots  mark  the  places  for  the  exit  of  nerves. 

Fig.  78.  Long  section  through  the  abdomen,  Stage  G,  showing  the  pit-like 
invaginations  behind  appendages  VII  and  VIII. 

Fig.  79.  Operculum  and  anterior  gill-bearing  appendages.  Stage  I.  (VII  and 
VIII  should  read  VIII  and  IX  respectively.) 

Fig.  80.  First  and  second  gill-bearing  appendages.  Stage  L. 

Fig.  81.  Longitudinal  median  section.  Stage  L,  in  which  the  connection 
between  the  mesenteron  and  proctodaeum  is  not  yet  made. 

Fig.  82.  Longitudinal  section.  Stage  I. 


266 


KINGSLEY. 


EXPLANATION  OF  PLATE  XII. 

Fig.  70.  Section  posterior  to  that  shown  in  Fig.  69.  The  sternal  arteries  have 
reached  the  nervous  system  and  lie  upon  it.  Beneath  may  be  seen  the  anterior 
extension  of  the  ventral  part  of  the  neural  artery. 

Fig.  71.  More  posterior,  showing  that  the  neural  artery  is  paired  above  and 
below  and  also  showing  connection  of  dorsal  and  ventral  parts  of  neural  artery. 

Fig.  72.  Horizontal  section,  Stage  L,  through  proventriculus,  sternal  arteries 
and  anterior  end  of  mesenteron  with  ducts  of  the  hepato-pancreas. 

Fig.  73.  From  the  same  series  as  73,  but  passing  through  heart  and  showing 
the  bifurcation  for  sternal  arteries,  an. 

Fig.  74.  Section  from  same  series  as  Figs.  69-71,  passing  through  fifth  pair 
of  legs. 

Fig.  75.  Through  the  abdomen.  Stage  L. 

Fig.  76.  From  the  same  series  as  Fig.  74,  showing  the  connection  of  sternal 
arteries  with  the  heart  and  then  splitting  for  the  stomodaeum. 

Fig.  77.  Reconstruction  (wax)  of  the  anterior  end  of  the  heart,  sternal  arteries 
and  neural  artery.  Stage  K.  At  the  left  is  shown  the  cavity  for  the  ventral  cord  ; 
the  dark  spots  mark  the  places  for  the  exit  of  nerves. 

Fig.  78.  Long  section  through  the  abdomen.  Stage  G,  showing  the  pit-like 
invaginations  behind  appendages  VII  and  VIII. 

Fig.  79.  Operculum  and  anterior  gill-bearing  appendages.  Stage  I.  (VII  and 
VIII  should  read  VIII  and  IX  respectively.) 

Fig.  80.  First  and  second  gill-bearing  appendages.  Stage  L. 

Fig.  81.  Longitudinal  median  section.  Stage  L,  in  which  the  connection 
between  the  mesenteron  and  proctodaeum  is  not  yet  made. 

Fig.  82.  Longitudinal  section.  Stage  I. 


.Jouriitil  of  Moi’i>Iiologi)  \ol.  VIU. 


pt.xrr. 


268 


KINGSLEY. 


EXPLANATION  OF  PLATE  XIII. 

Fig.  83.  Horizontal  section,  Stage  I,  to  show  the  relations  of  the  mesenteron 
and  lobes  of  the  hepatopancreas. 

Fig.  84.  Transverse  through  an  embryo  of  Stage  L,  to  show  the  conversion  of 
yolk  cells  into  the  epithelium  of  the  alimentary  canal. 

Fig.  85.  Longitudinal  section  through  the  junction  of  stomodaeum  and  mesen- 
teron, Stage  L,  before  the  connection  of  the  lumens,  to  show  the  transformation 
of  yolk  cells  into  the  columnar  epithelium  of  the  midgut. 

Fig.  86.  Late  Stage  L showing  the  first  hepatopancreatic  duct. 

Fig.  87.  Late  Stage  L through  the  third  pair  of  appendages. 

Fig.  88.  Longitudinal  section  through  the  oldest  larva  studied,  showing  the 
junction  of  mesenteron  and  proctodaeum.  The  section  was  not  quite  median  and 
hence  cuts  off  the  folds  in  the  proctodaeal  region. 

Fig.  89.  Section  of  a leg,  Stage  L,  to  show  the  nerve  surrounded  by  the 
artery. 


268 


KINGSLEY. 


EXPLANATION  OF  PLATE  XIII. 

Fig.  83.  Horizontal  section,  Stage  I,  to  show  the  relations  of  the  mesenteron 
and  lobes  of  the  hepatopancreas. 

Fig.  84.  Transverse  through  an  embryo  of  Stage  L,  to  show  the  conversion  of 
yolk  cells  into  the  epithelium  of  the  alimentary  canal. 

Fig.  85.  Longitudinal  section  through  the  junction  of  stomodaeum  and  mesen- 
teron, Stage  L,  before  the  connection  of  the  lumens,  to  show  the  transformation 
of  yolk  cells  into  the  columnar  epithelium  of  the  midgut. 

Fig.  86.  Late  Stage  L showing  the  first  hepatopancreatic  duct. 

Fig.  87.  Late  Stage  L through  the  third  pair  of  appendages. 

Fig.  88.  Longitudinal  section  through  the  oldest  larva  studied,  showing  the 
junction  of  mesenteron  and  proctodaeum.  The  section  was  not  quite  median  and 
hence  cuts  off  the  folds  in  the  proctodaeal  region. 

Fig.  89.  Section  of  a leg.  Stage  L,  to  show  the  nerve  surrounded  by  the 
artery. 


Journal  of  Morpholoijij  I bl.  I 'HI. 


J.S.KDtl. 


Frxr.kr^'-M. 


